Can a black cow and a bull have a white calf?

No. Black is a dominant colour, no matter what colour the bull is. If the bull is white, the calf will be grey. If a bull is black, the calf will be black; if a bull is a blue roan, the calf will be most likely a darker roan or even black; if a bull is red or brown, the calf will be black; if a bull is yellow or orange the calf will be either grey or black.