The eye color follow quite well mendelian inheritance rules...
Probably your parents are heterozygote recessive for the blue color, hence the children have 25% of probability of having blue eyes.
Let's say color of the eyes has 2 alleles "BLACK" for the dark eyes and "blue" for the light eyes. Your parents are probably both BLACK-blue, BLACK-blue. They show the dark eyes because that allele is dominant over the blue one.
During recombination all the possible combination can come out:
- BLACK x BLACK=25% resulting color=dark
- BLACK x blue=25% resulting color=dark
- blue x BLACK=25% resulting color=dark
- blue x blue=25% resulting color=light
Check out the wiki page about mendelian rules of inheritance if you wanna know more!!
Of course in realty the situation is much more complicated, however you can still approximate the results to mendelian rules...