Please refer to the wealth of information available at the CDC (Centers for Disease Control) website (
http://www.cdc.gov/hiv/resources/factsheets/oralsex.htm). This particular link focuses on HIV but there are links to other STD info as well.
Now, the difficult truth in the hobby is that WE ALL KNOW that CBJ is far safer than BBBJ for all parties. But most gentlemen and many reputable providers have elected to accept the inherent risk involved in BBBJ. The concensus among the scientific community is that BBBJ has some risks but far less than penetrative anal or vaginal sex. HOWEVER, much of the evidence is anecdotal because, lets face it, very few people ONLY have oral sex. These days people do it "every which way" imaginable, sometimes protected, sometimes not. So when an STD is contracted it is virtually impossible to link it to one specific sex act.
There are biological and anatomical factors which make BBBJ somewhat safer than penetrative sex. Once body fluids pass through the mouth and enter the digestive system (yes, thats gross to think about), most bacteria and viruses are neutralized by acidic gastric juices. This level of protection is not present with other types of sex.
We can give ourselves a false sense of security by offering BBBJ without CIM but we all know that bodily fluid is still exchanged during that act. Also, the simple skin to skin contact presents risk in and of itself.
The bottom line is that you have to decide what risk you are willing accept for a great BJ. There are obvious stupidities that could put you at greater risk like receiving a blow job if you have any sort of cut or abrasion or if your partner has any visible sores. But that's not inherent risk, that's plain stupidity. Like LAVixian said, the only way to avoid the risk is abstinence. You, like everyone else in this hobby, have to make your own personal decision.
(And yes, in my "side job," I'm a health care professional)