Science genetic question ?!?

The black horse is a hybrid. Hybrids are heterozygous which in this case the black horse is Bb. When crossing the two Bb x bb you will get 50% bb. If the black horse was purebred which is (BB) you wouldn't have the possibility for a brown horse (bb) since BB X bb gives all Bb which are black. SO it has to be hybrid to have a brown foal.