I have looked far and wide on in as many places as I can reliably trust for information regarding the answer to your question, and unfortunately, I came back with no less than fifteen answers, none of which corroborate each other. It seems that this question may never have a concrete answer. Everyone who tries to answer this question does so with such authority that most people are satisfied, and it's a touchy subject, so most people never go to look anywhere else for a corroboration. Some people even go so far as to cite sources as being the "first time it was put on paper" but never actually produce the evidence or at least show some copy/pasta. I'm not going to do that to you. Well, not really.
Okay, I'm totally going to do it, but at least I hope to entertain you, and I'll be citing something much more amusing. In 2006, comedian Robert Wuhl, whom I swear you've seen in a movie somewhere but would not know his name if you saw him again, starred in an HBO special entitled Assume the Position with Mr. Wuhl. In it, he taught a class that centered on the theory that "History is Pop Culture." The basis of this theory was that if you heard about it in history class, it's because the people reporting current events back in the day were just as susceptible to popular opinion and interest as the big media networks of today are. This brings up an interesting point, in my opinion: lay off the media giants and their "control" of the industry; you don't have to get your news from them, and giving the people what they want to hear is just the way it works, not their own little agenda. And while we're on the subject, if you don't like your news, get it from somewhere else. It may not be more reliable, but it will at least make you happier.
But, I digress. The point is, in this little class that Mr. Wuhl taught, he gave us the legend of where using the middle finger as an insult came from. However, at the end of this explanation, he asked a question and subsequently answered it. "Is it true? It doesn't matter! Because when the legend becomes fact, print the legend." Unfortunately, in this case, the case of the etymology of the term "blowjob," there is no legend. It's in common use, just as the middle finger is, but the story of how it came to be known as such is lost to time. So, instead of giving you an answer I know to be true, I'm going to give you the one that I find most interesting, and one closely related to it.
Both of these explanations supposedly come from Victorian era England prostitution. The simpler of the two, and one that lacks a bit of punch in my opinion, is that "blow" was shortened into it's current form from "below." A "below-the-belt-job" as it were. Meh. Okay, I can see that, but then why aren't the perfectly named "hand jobs" lumped into the category of "below jobs"? I didn't like the explanation, but it is closely related to the one that I do like, and it's as close to corroboration as I can get on this.
The second explanation involves sailors. As any pirate movie you've seen within the past ten years will show you, sailors back then were often very nasty individuals who went for long stretches of time without sex. This is why most of the prostitution trade catered to this class of individual: they were dirty, ugly and very hard up. In a way, they were the perfect client for a criminal that provided the world's oldest paid service. The rich and pretty didn't need their services, and risked imprisonment if they were caught doing business with them, and it was much easier to just meet some nice tart somewhere and throw on the noble charm, so the fine and upstanding rarely utilized their services. The truly desperate simply couldn't afford them. Sailors, on the other hand, got paid, and relatively well. However, that didn't always make up for their unwashed exteriors, and while letting some filthy dirt ball gyrate on top of you for a while is bearable when you can breath through your mouth and not your nose, that doesn't work when there's something in your mouth. Telling a john that one wouldn't do something, however, was usually a little risky, since one was doing something illegal and therefor couldn't go to the police if one wanted to report having one's face broken in. So, a few wise and enterprising ladies of the evening went capitalist on them.
In Victorian England, sailors who had been granted shore leave but who had not completed their time with a particular ship were given an allowance of their overall pay to enjoy their time ashore. This was to keep them from blowing their earnings before they finished their time with that ship, which usually meant that there were a few less murderous wives when they reached their home ports. Well, the dock masters of each port required some quite intricate lists from captains that made port. Information in these lists included number of crewmen, duration of stay, nature of cargo, business in town, and more importantly to this story, the amount of shore allowance given to each sailor. The smart prostitutes would go to the dock master and ask him for this amount, just so they would know how much dough the sea dogs were carrying around. When it came time to get down to business so to speak, and the john would ask for fellatio, the girl would quote him a separate price for the service, and it just so happened to coincide with nearly or exactly the amount of his shore leave. Ergo, to get his dick sucked, he would have to blow all his money on the job.
Both of these explanations cite English court records as being the original public instance of the term's usage, but they never give exact words or show where they can be looked up. However, the important things to keep in mind are that 1. They both cite the same source, specifically mentioning Victorian London, and 2. It doesn't matter. This is the legend, and I'm printing it.
If you have a question for the demon, send it to askthedemon@gmail.com, and remember: never stop asking questions.
