Stallion is a man who fucks a taken lady, that is, a girlfriend or wife of another man with their permission and/or encouragement, but without humiliation, denial, cuckolding, sissifying, bi play e.g. fluffing, or otherwise demeaning the man. A black stallion is a BBC who enjoys fucking a wife with the husband knowing and/or watching, where they might high-five each other for making the wife climax, perhaps take turns on her.
Bull is a dominant man who fucks a girlfriend or wife mostly for the purpose of humiliating and demeaning her man. The goal is to point out how much better the bull is at lovemaking, how superior his equipment is, how much more satisfied she is with the bull than with her own man, and thus the man must assume a subservient role to the bull, perhaps being made to fluff him or clean him or her, or wear panties.
The basics of fucking a woman who is in a solid relationship is the same, but the purpose is different.
A husband who is a stag wants to show off his sexy woman and wants others to appreciate her and thank him for sharing her. It's as if he has a Ferrari that he lets his friends test drive now and then. This relationship is stag/vixen and they seek stallions. Here on B2W they seek Black Stallions.
A husband who is a cuck enjoys being denied his own woman "without" his consent, while simultaneously other men DO get to enjoy his woman. This is cuckold/hotwife and they seek bulls. Black superiority porn is almost entirely cuck/bull, as is black breeding (a form of humiliating her man by forsing him to raise a baby that is not his but conceived after his relationship with her began).
I know many couples who enjoy cuck/hotwife/bull play and consider them good friends, but the vast majority of couples and encounters I've seen or participated in have been stag/vixen/stallion.
As wife sharing progressed from mostly online fantasy to a mainstream activity that a lot of couples are doing behind closed doors, the cuck/bull realm of fantasy gave way to the stag/vixen reality, as most men, even husbands of QOS women, are alpha or mostly alpha, are straight, and don't intend for their wife-sharing to be a humiliation. Likewise, most of the BBCs I've met have first asked to make sure my husband is stag and not cuck because they really weren't into seeing a husband in panties who wanted to lick their member. But no offense to the cuck/hotwife couples, everyone is entitled to enjoy their own thing.
My husband and I are stag/vixen and seek black stallions.
I use the term hotwifing to describe my hobby, but in reality I consider it vixing = the non-humiliating wife-sharing fun that couples partake in.
If this helped clear it up, please give this a like so we can use terms that describe ourselves more precisely.
I think they're used interchangeably Gordo. Based on what part of the country you're in, the individuals backgrounds, etc as to how they perceive a dominate Alpha sexual male. Of course many will say "cock size" ... but then you get into argument as to how big is considered large enough for bull or stud or stallion status. Its just assumed that an alpha male of these titles has an above average cock ... if he drops his pants and pulls out a 5 inch pecker ... well, you can just imagine the disappointments he'd face. 
