I answered previously, but did a little research since then.
There are two well-argued schools of thought.
Both explanations share the view that interpreting the word blow figuratively is the key to understanding blow job. Then you are in striking range of deciphering the steamy slang. Now, you just have to decide which figurative meaning of blow the word refers to.
One explanation ties the phrase to the jazz slang blow meaning 'to play an instrument'. From there you get the semantic connection of working a tool with some skill, presumably involving the mouth, and there you go.
The second school of thought proposes that the blow in blow job indicates the climax, and is also related to the expression "to blow off steam." Although the phrase blow job dates only from the 1940s, there is a reference to blowing (someone) off, roughly equivalent to the modern phrase "getting (someone) off," in David W. Maurer's 1939 glossary, Prostitutes and Criminal Argots.
In either case, the word was used primarily in relation to prostitutes, or as a menu item in brothels, until the 1960s. By the 1970s, the use was widespread in the U.S. and the U.K. Whatever its origin, it can now be shortened to only blow, and is often used to make combined forms, like blow book 'a pornographic magazine' or blow buddies 'friends who engage in oral sex'.
Also puzzling to me was the use of blow (or blo) to refer to cocaine. I mean, when you "blow snow," you're really inhaling, right? This blow seems to be related to a euphemism for smoking, and was probably borrowed from a more logical phrase, to blow weed or blow pot, meaning 'to smoke marijuana'.
Hope this helps.........
- Response by usmale
, A Player, Male, 56-65, Civil Service