I used a condom giving BJs to my late partner, whom I knew was poz. He also topped me with a condom.
Before the condom went on I would tongue his shaft & balls, avoiding the urethra. If precum appeared I went for the condom.
I don't like the balloony taste of latex condoms, nor the stuff they put on them, but they're supposedly a better barrier against HIV than non-latex. So for BJs I'd get unlubed, non-spremicidal latex ones when I could find them, and when I couldn't he didn't mind if I wiped him down with a damp washcloth first after I had the condom on him.
With my current neg partner I've never used a condom for a BJ.