The phrase "blow job," for oral-genital sex performed on a male, is surprisingly new in terms of its widespread understanding and usage. It started to appear in slang dictionaries in the 1960s, around the time pop icon Andy Warhol released his film Blow Job, containing several explicit depictions of the act. Earlier the term had been used by college men, prostitutes, and printed in underground pornography, but it was not yet commonplace.

To many Americans in the 1940s and 1950s a "blow job" was a faster-then-the-speed-of-sound "jet airplane." It took off and gave everyone nearby a "blow job." The Thesaurus of American Slang (1953) records an example of this usage from an issue of the San Francisco Examiner in 1945: "A P-59 jet propelled Airacomet, affectionately called the 'blow job' by flyers, will make several flights in 1946."

Linguist think the sexual connotation of "blow job" evolved from "blowoff," an expression meaning to finish off, to climax, to end. "Blowoff" in this sense is related to "blow off steam," to put an end to a emotionally frustrating experience. When a prostitute gave a client a blow job she was helping him "blow off" the steam of sexual arousal. In the 1930s, street-walkers offered oral sex with the phrase "I'll blow you off." It suggests 'I'll cool you down,' 'I'll release your steam.'

Some linguists think the term "blow job" evolved gradually from an eighteenth century European name for a prostitute, blower. A popular name for penis at the time was "whorepipe," and it is easy to see how the woman who played the instrument came to be called a "blower." But was the act called a "blow job?" There's no indication of that.

Today the word is commonplace, uttered as often by women as men. In the following limerick, a widow has just had her cheating husband's body cremated and is about to dispose of his ashes:

A bitter new widow, quite tough,    
To her mate's ashes said in a huff,
"You've diddled young girls,
Never brought me no pearls,
And wanted me to blow you - so puff!"

Source