Every person is a world unto itself. The interaction of two creates a special synergy, and when it comes to three it is even more difficult to give an advice, because there is no universal formula. At first, your situation seems to be quite usual. A man excited by the idea of "sharing" his wife, but then discovers that he has opened the Pandora's box and he cannot deal with the situation because reality has little to do with his fantasies.
The problem is, if your husband is not a truly omega male, for him could be really hard to accept that other man is better than him in sex. Maybe he expected the bull just to be a "complement" for your sex life and now he considers the black stud as a rival that has overcome him. This situation can easily affect his self-esteem. Most of the advices in this thread have assumed your husband is an "authentic" cuckold, someone who enjoys with humiliation, but he can also be a person that has unsuccessfully tried with a kink.
If you really love him, you should behave to give the impression that your relationship with your bull is just sex. You should make your hubby feel special in some way. Maybe a solution is mocking of your bull, or whatever you can imagine.