I would try mixing certain groups of the goats to begin with. Ie, milkers from one herd with goatlings from the other, the other set of goatlings with a batch of kids, the other kids with the other set of milkers. The older age group in each case will soon extort their dominience over the younger group, and they will integrate, then you can add in the bigger groups together gradually.
Colour prejudice definitely exists- but it tends to exist for example if you keep a herd of white goats for 5 years, then bring in a toggenburg type. Had it happen here- we had a mixed herd of AN's and BT's for probably about 10 years, and then my mum got in a BT/BS cross kid because it was out of her male, and she was convinced her goat was going to have twin males. I had a AN kid who was 6 weeks old who had never seen another kid before, and she loved her new white companion, just because it was someone the same age as her. Every other goat in the herd HATED the white kid, and went out of their way to beat her up. It actually got to the point where Skye, the AN would try and throw herself in between the adults and Libby (the white kid).
Oh and my mums goat kidded in april with twin females, so she got female kids out of her billy after all! Libby was also cleverer than my AN kid, and taught her to jump out of the field! To add insult to injury, she never had a female kid. But since then, we've kept a white goat in the herd (not necessarily through design).
Sounds like MGoM's goats are used to being a white goat herd and dont like black goats!
Beth