blow it = fail at something把事情搞砸了。
A: How did you do on the history exam?
B: I think I blew it! There was a section on the Civil War, and that''s the chapter in the book that I studied the least.
A: Well, at this point you can''t really be sure that you completely failed the examination. You must have done okay on the rest of the test.
B: Perhaps I didn''t fail the entire exam, but I''m sure that I didn''t do well.
(Source from koolearn.com)