Pure white or black ponies don't exist - same goes for all shades of grey. For breeding black or white ponies, mix grey with colors. You're mostly lucky. :)
"Dimmi, mente amantissima mia..." ~ Leonardo da Vinci
I have one triple white stallion, six triple blacks, and 35 triple greys you're welcome to send breeding requests to. And a lot of other colors, too, of course.