BBBJ is a low risk activity because HIV is present in the salvia at really low levels, so theoretically it's not sufficient enough to cause an infection. However, a HIV+ individual's semen or vaginal fluid or blood from a mental has a higher concentration of the virus and poses a greater risk if the person giving oral has a cut/sore in their mouth or bleeding gums, but again its still low risk activity. Reason being, the lining of the mouth and throat are "considered" to be less resistant to the virus.
BBFS lol is a different beast. Statistics show that men are more likely to pass the virus on to women because of the stretching and tearing of the walls (through Greek and vag). When those walls stretch, tearing allows many microscopic (visual if blood is present) openings for the virus to seap inside the tissue. This is considered an high risk activity, bb Greek being the greatest of the two.
Ru
Health advocate
http://aids.gov/hiv-aids-basics/