The technical definition of blurb is: “A brief descriptive paragraph or note of the contents or character of a book, printed as a commendatory advertisement, on the jacket or wrapper of a newly published book.”
But the first use of the word, in 1914, by G. Burgess, defines blurb as “a flamboyant advertisement; an inspired testimonial.”
Blurbing is its own art. There are people who blurb books in magical ways, paring down an entire book into a few glowing sentences. I can think of quite a few writers who do this with real flair: Joyelle McSweeney, Brian Evenson, Gary Lutz, etc. They make blurbing seem easy, effortless. I call them magicians.