I have been bold enough to ask a stranger. I make small talk with them first (directions, any general conversation) to feel them out and if they are friendly. After I qualify them to see if they are friendly enough or not a threat, I say that I'm not from the area and I wondered if they could tell me if there is anywhere they could recommend to go for some fun. I feel by telling them that I am not from the area, there is no threat of anyone finding out. They usually say that there is a strip club near by or something of that nature. I then clarify and say, "well, actually, I'm looking to give a blow job".
Most times they respond that they don't know and sort of smile. If they smile and are somewhat friendly, I will pursue it and ask them if they would be interested. Then I follow up and say that nobody would ever find out and that no reciprocation was wanted.
It's surprising how many straight guys will agree after some persuasion