Originally Posted by j3rkules

Another origin theory is that in the 1930s, a sailor taking shore leave in London would have to pay a prostitute dearly for the (then) exotic service, usually all of a sailor's leave allotment (which the prostitutes somehow knew to a precise ammount), an act commonly referred to as "blowing all one's cash," making the service a "blow it all job," or "blow job." Another story says that it comes from a perception that fellatio less often causes orgasm than traditional intercourse, causing many cases of blue balls - a blue job - and the word blue mutated into blow.

This is what I found, not sure if any of them are accurate.