Black is recessive. Two black goats can never give you a white baby, but two white goats can and do, as you have seen!
Your black buck is what is known as a Sable Saanen. He is all Saanen, but Saanens often carry color genes. Sables are becoming popular and (I've been out of the ADGA loop for a while) were well on their way towards becoming a sanctioned ADGA breed in their own right the last I heard. They are popular and in demand in some circles, and for good reason: Sables have all the same productivity of their breed without the skin cancer risk (besides, they're pretty!). Something else- Sables can come in just about any color, not just black.
http://www.sabledairygoats.com/
http://nationalsaanenbreeders.com/00klisse.html
Klisse Foster raises both types of Saanens, above you can see her National show winning Saanens and Sables! She is one of the top Sable breeders.
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Let's say that a black buck (and black bucks aren't very common)
was running loose somewhere, jumped the fence, and bred your doe while you slept unawares....your doe would have had a pure white kid unless she was carrying the gene for color. Black is like that- it has to come from both sides or you just cannot get it at all. I know this because I have black goats and breed for solid black- it wasn't easy to get....I had to do quite a bit of linebreeding in order to get it from my (already colored) Alpines.