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: Oral sex - Wikipedia, the free encyclopedia...
"The common slang term blow-*** 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-***" indicates the climax of the penis (compare to blowing of a volcano). The phrase "blow-***" originated in the 1940's;[7] at the time, it was also used by Allied pilots as a slang term for jet aircraft.[8]"