and expanded a little in the Macmillan Dictionary:6 informal completely bungle (an opportunity)
6 [TRANSITIVE] INFORMAL to destroy your own chance of succeeding, or to waste a good opportunityI've completely blown my diet with that piece of chocolate cake.
blow it: We were in with a good chance for that contract but you've really blown it now!
Hi natkretep,Henryhiggins's sense (blow = leave quickly) is available only in AmE. I don't think I've heard it in BrE.
I think MarFish's sense is the correct one....