Blow It Meaning

(idiomatic) to fail at something; to mess up; to make a mistake.

Example:   I blew it and forgot to start the spaghetti, so I had plenty of sauce and no pasta.
2014, Daniel Taylor, "World Cup 2014: Uruguay sink England as Suárez makes his mark," guardian.co.uk, 20 June:
  Hodgson’s team attracted a certain amount of sympathy and understanding after the Italy defeat but it was beyond them to play with the same attacking panache and, if there is to be a feat of escapology, it will need an almost implausible combination of results and handouts in the final games of Group D. More realistically, they have blown it in their first week.