black bull is not slang and is not a male cow in cuckold sex.

Cuckold sex is the technical term for when there is at least 3 people, two are having sex and the third is forced to watch them.

black bull is the position name / technical term for the very large in every physical way, black man that is having sex with one of the people while the other person is forced to watch.

just like the term White Bull is the position name/ technical term for a very large in every physical way, white man that is having sex with one person and another is forced to watch them.

in short thats that cuckold sex is. one larger person is performing rough sex on another while a third person is forced to watch. in some cases it can border on rape in other cases it can be a gentle form of rough sex, but the key elements are a committed couple in a relationship and a very large third person. if that very large third person is black their position is called black bull and if they are white their position is called white bull.

(an easy way to think about this is take it out of sex act and into a sport, sports have their own position names. so does having sex with someone)