Hello, I'm new here and just want to add my experience and making my case a generality.
I'm not sure that the Bull is a type of man, obviously some men are more dominant than others, but I think this black bull stuff come more from the fact of being with a white woman.
In both Western and African culture the man is the leader, he is superior to his wife within the society and expected to be dominant. The white woman has to make cum the white man she is submissive to her husband.
But with black men in the western world, for long time, even if racism is decreasing now, the white woman was superior to the black man due to racial bias. So in a relation with a black man, to readjust the relation to something "normal", the white woman has to accept not only to submit but to accept some kind of act that both agree as "humiliation". Otherwise the black man will be inferior in the couple due to the advantage of the white woman in the western world.
This is the drama of the white woman / black man relation, if the white woman don't accept the humiliation in the bedroom the black man cannot feel that he is in a normal Man to woman relation.
Once again jut my experience and I totally accept that what I say can be limited to me and my past lovers.
By the way sorry for my English if it makes it difficult to understand or not clear