When a black-fur bunny and a white-fur bunny reproduce what is the fur colour of their off-spring

I left my rabbit with black fur with some one else's rabbit which has white fur... will the offspring have brown fur?

The bunnies will either be white or black. Most probably some will be white and some black. That is because the fur color is determined by related genes which transfer as whole unites. It there are more than one gene that determine the fur color then the rabbits might be gray, but if you take a look at dogs, litter of black and golden retrievers is not gray, some are black some are white.

For example in people; children of one white and one black parent would most probably be of skin color in-between, because it is determined by more than one, but the eye color would be either mother's or fathers, eye colors do not mix.