If a SP is offering BBFS it is because she has nothing to loose.
Ask yourself if you are really confident "playing" Russian Roulette that when you pull the trigger, there is no bullet in the chamber.
I remember (several years ago) my buddy telling me about a dude who knew he had HIV going around having unprotected sex with females and men. The local media reported that the health agencies reached out to a couple of dozen potential victims to have them tested.
As as someone said earlier in this thread: if a dude (or lady) engages in BBSF with an SP, he's either a moron or has a death wish. I'd add to that warning: the dude may know he's HIV positive and trying to spread and kill others -- (misdirected) Revenge!