Well, I thought it was like human colouring. The darker you get the more skin pigment you have in your skin (melanin). So, without this skin pigment we'd all be albino people. I thought of it kind of like a "default" colour that was changed with the genetic predisposition of how much melanin our bodies make. I wrongly assumed the same was for zebra's, that the pigment was the dark colour not the light colour. But that's totally cool that it's the reverse. Actually I think that's rather fucking awesome. XD Lol!
According to embryological evidence a zebra is black with white stripes, the white being a lack of black pigmentation. Even if they have white bellies, their muzzle, inner ears and underside of their tails are black