The black pitbull is homozygous for black (BB). The white pitbull is homozygous for white (WW). When they are crossed, the F1 generation will all be heterozygous for black and white (BW). This means that they will all be black, but they will carry the gene for white.
When the F1 generation is interbred, the F2 generation will produce a variety of coat colors, including black, white, and blue. The genotypic ratio for coat color in the F2 generation will be 1 BB : 2 BW : 1 WW. This means that 1/4 of the puppies will be homozygous for black, 2/4 will be heterozygous for black and white, and 1/4 will be homozygous for white.
So, if you cross two blue nose pitbulls, one of which is all black and the other white white, the resulting puppies will be both black and white.