Polar bears are white in color and white color is not a good absorbant of heat. Why then didn't they evolve a black color to absorb more heat?
Polar bears do not have white fur - they don't have pigments in their fur hairs which makes them clear and translucent. The outer fur consists of hollow fibres, which shatter a part of the incoming light (the visible part) which makes them look white. This gives a very good camouflage in a white environment.
It was initially thought (see reference 1) that the hollow fibers work in the same way as fiber optic cables and direct the UV part of the light (the visible was thought to be shattered) towards the black skin of the bears where it generates heat, but this hypothesis has been disproved. The UV part of the light is simply absorbed by the bears outer fur and doesn't reach the skin (see reference 2 for details).
The hollow hair helps with the insulation of the skin and can change the color due to environmental influences. The tend to get yellow after feeding on seals due to the fatty substances from the animals. There have also been reports from zoos that algae lived inside the hollow part of the fur making the animals more grey and sometimes even green (see reference 3).
Bears initially were dark. But in snow it is really poor camouflage, their victims will see them to early. So they evolved to white color.
They have a lot of fat and good fur to keep the temperature, they don't really need more sun energy. They need more food.
In the same way - in the forest all bears are dark - for camouflage.
How much energy could an Arctic-dwelling creature be expected to capture from sunlight? I think not very much. How much can it gain from having coloration that allows it to capture prey more easily? Quite a bit. Also, given the size of the polar bear, they will probably be generating more than enough waste heat from metabolic activity. Indeed, if you do a little research, you might discover that the polar bear's problem is, indeed, getting rid of heat:
In bears, the basal metabolic rate varies with climate and season; Polar Bear has highest BMR. Overheat easily when running; can't travel any great distance at speeds more than a walk. Low surface-area to volume ratio favors heat retention. Tendency to overheat enhanced by layer of fat 11 cm (4.5in) thick. Young cubs up to 6 months old, with little body fat, are susceptible to cold. Use more than twice the predicted energy for moving at a given speed, perhaps due to bulky body. Fat and fur both insulate. As long as bear isn't exposed to wind, body temperature and metabolic rate remain normal at -37 degrees C (-35 degrees F).
Of course, even if the polar bear was a perfect absorber, it could get at most ~1kW/m^2 of energy from the sun, and that only on a clear day in the summer. And that heat would be on the outside of its insulating fur & fat.
So there's the answer: a strong selective pressure towards white, little or none towards black.
Re waste heat: What is meant here is that when the polar bear (or any animal) consumes food, part of it goes to producing heat as an unavoidable byproduct of metabolism. Larger animals usually have to get rid of excess heat, thanks to the surface/volume relationship mentioned in the comments.
For a reference, try thinking about this: http://en.wikipedia.org/wiki/Thermoregulation And remember that not all answers are written down in a book (or web site, these days). Sometimes you have to actually think and reason out your own answers :-)
Added: I think the problem with this question is the assumption that polar bears (or indeed, any boreal mammal) need to absorb heat. Maybe that's down to anthropocentrism, since humans evolved from ancestors that lived in tropical climes, and so have adaptations (like sweating & nearly hairless skin) for cooling. But if you read the links, you discover that more often the problem is getting rid of excess metabolic heat, and most boreal mammals seem quite comfortable in what we primates consider cold. (I write this after coming in from playing with my two dogs, who love to run around in the fresh snow.)
You might also think about why almost all (non-domesticated) mammals have evolved protective or camouflaging coloration. (The only exception I can think of offhand is the skunk.) Indeed, a number of boreal mammals have evolved seasonal color changes (e.g. http://srel.uga.edu/outreach/ecoviews/ecoview130106.htm ), having darker summer colors (like their more southerly relatives), but turning white in winter. That suggests that the benefits of camouflage strongly outweigh any possible benefit from solar heat gain.
In addition to camouflage, consider that during the colder parts of the year, when they need the heat the most, the nights are longer than the day.
Darker fur would radiate more heat away from the body than the lighter fur.
So it's possible that they did evolve the white fur for heat - but in their environment, the coldest part of the year heat retention is more critical than heat gathering, and this would bias towards white fur, not black.
Although black is a better absorber of (heat) radiation, it is also a better emitter too. As Polar bears live in a cold environment, having black hair would cause them to radiate their body heat away more quickly. Their colour helps them to retain heat better - dark objects equilibrate to the temperature of their surrounding faster, lighter objects more slowly. If they were darker, they would require more food to survive.
Their black skin is irrelevant as the light has already been scattered/reflected away by the white/colourless hairs - as they look white, it's the reflected light that you are seeing.
Why then didn't they evolve a black color to absorb more heat?
This assumes that evolution is an active process, working by organisms' will or need.
Do not forget that the theory of evolution works from generation to generation, by random (imperfect and chaotic) mutations, most of which are unfavorable.
The polar bears did not evolve a black color because they can't decide that. If you believe in evolution, and you believe that a black color would draw more heat, increasing the animal's chances of survival over its white peers (so much that the white peers would eventually die off), then we are simply waiting for the black color to come around. Evolution is never finished.
This is a brief expansion on DrAlchemist's response; one that I intended to make myself, but saw that s/he had already covered the basics.
To see that black is a better emitter of heat we have to leave biology and go to physics, specifically quantum mechanics, even more specifically black body radiation. Like most things, absorption and radiation are equally balanced when it comes to a given material/color. A white object is slow to absorb heat and a black object is quick to absorb heat, as we have likely all seen demonstrated in elementary school. What my teacher (and likely yours) did not mention is that the white object cools slowly when taken out of the sun, but the black object will cool much more quickly. Black heats up quickly, and radiates just as quickly when the temperature differential inverts.
As a mental exercise, consider an idealized object that absorbs electromagnetic energy perfectly, called a black body. Turn a heat lamp on it, and it will absorb all the heat it receives. Turn the lamp off and, as in the argument above, it will become a perfect emitter of the radiation it absorbed.
I think an important point hasn't been mentioned yet.
Why then didn't they evolve a black color
The changes introduced by evolution initially are random. Only the persistence of those changes over generations can be thought of in a "smart" way.
Technically, evolution can lead to some things getting worse! For example, if mutations improve a creatures eye-sight but at the same time worsen it's hearing, both traits may end up surviving assuming the improved eye-sight makes more of a difference than the worsened hearing.
Well, Polar bears do have black skin; But, living in the harsh arctic conditions they've adapted several traits such as thick layer of blubber, wide paws etc. But their most peculiar trait is having a white fur coat; unlike other bears, polar bears have evolved to have white fur with hollow hairs which easily allows their black skin to absorb the radiations from the sun. So there's no problem with the heat intake.
Now, the reason behind their white fur lies in their quest to food. The white fur enables them to easily hide/blanket in their surrounding (the white masses of snow) i.e. it helps them in camouflage.
And, I guess that's it! Thanks!