I'm not sure it makes sense, but are zebras black or white below the surface?
It is not like someone painted a zebra one color (say white) and then painted lines of the other color (say black) on top. But it does not mean we can't give some thoughts on the question.
What causes these patterns?
The colors are caused by pigments. Melanocytes either produce black pigments or do not and this cause the alternation of white and black stripes. The exact developmental pathway for producing these stripes is still very much unknown. All we know is that several environmental variables seem to affect this pattern. The most important correlate found so far is temperature, where higher temperature lead to darker and better defined stripes (Larison et al. 2015). This pattern can also be observed geographically (although there might have other important covariate driving this pattern, such as genetic typically)
So, are zebras black or white?
According to the Brevard Zoo, under their white or black fur, the skin is always black. This, I suppose, might lead others to rather claim zebras are black with white stripes.
From @iayork's answer, it appears that embryos are first black and then white tripes appear. This an extra reason to claim that zebras are black with white stripes. Note however, that I am not sure whether the skin of zebras are not first white, then turn black and then black fur and then white fur. Under such circumstance, one might want to argue that zebras are white with black stripes.
So if we take your question as phrased
[A]re zebras black or white below the surface?
If by "below the surface" you are referring to their skin, then they are black (according to the Brevard Zoo)!
Zebra embryos start off black and develop white stripes late in development. This is widely used to conclude that zebras are actually black with white stripes, and it's as good an argument as any for this essentially semantic question.