Believe it or not, according to yahoo answers:
The name came from one of 3 possibilites: Victorian slang for a "below-job" (as in below the belt), for the volcanic reaction of the resulting orgasm, or a facetious use of "blow" as the opposite of "suck".
According to Wikipedia:
http://en.wikipedia.org/wiki/Oral_Sex#Fe…
"The common slang term blow-job is a misnomer, as blowing is not normally part of fellatio. In fact, blowing air into a partner's penis is an unhealthy practice. It is most commonly thought to be a facetious use of "blow" as the opposite of "suck". It has been suggested to be a corruption of "below-job", allegedly an old Victorian slang term that was commonly used to describe the act. However, there is a third school of thought that believes the word "blow" in "blow-job" indicates the climax of the penis (compare to blowing of a volcano). The phrase "blow-job" originated in the 1940's;[7] at the time, it was also used by Allied pilots as a slang term for jet aircraft.[8]"