The thing is that with full service, there is no such thing as "safe". So while you can reduce the risk, it'll never be completely safe. I believe condoms have a failure rate of about 1/10.
Now given that I believe it's far more likely to contract a serious std from vaginal or anal intercourse than it is with oral, I'd probably say a bbbj is probably less risky than covered full service. Say it's a tenth less likely to contract an std through oral than it is vaginal, then the odds of catching an std through a bbbj would be even with catching one with safe FS.
Also, it would probably be much safer to give a bbbj then put the condom on for FS then it is use the same condom for both the bj and fs as has happened with most sps I've seen. A cbj is probably putting the condom at risk for wear and tear. Using a condom for each one would probably be much safer.
I'm not saying an SP should be obligated to perform a bbbj (unless they're adversting PSE) as putting a complete stranger's you-know-what into your mouth might not be the most comfortable thing. However, it's still relatively a safe thing IMO, at least as safe a covered FS and maybe even safer (I don't know the exact statistics).
Also, with COF/COB, most SPs would charge extra for this, or not do it at all and that's understandable a bit since like a bbbj, it's not exactly the most comfortable thing to do with a stranger. But it would have to be much safer given the aforementioned failure rate of condoms wouldn't it? It would be pretty much impossible to catch an std from COB whereas, even with a condom, CIP would still be somewhat risky. About one tenth of all clients who have CIP with an SP have probably done so with a broken condom so you'd actually think SPs would be encouraging COB, rather than refusing it or charging extra.