Because the black mouse is homozygous.
The black color in mouses is dominant, and the brown is recessive. When a parent is homozygous, their children need to have their gene. Because in this case the trait is black, it dominates brown in all children.
The table is BB×bb = Bb, Bb, Bb, Bb = 100% heterozygous genotype = 100% black phenotype.
