Context #1
Jim: So how did you do on your driving test yesterday?
Harry: Oh man, I totally blew it!
Jim: What happened?
Harry: I accidentally ran through a stop sign, so I failed it.
Context #2
Kathy: I have that big job interview later today.
Sara: Good luck!
Kathy: Thanks, but I'm so nervous! I hope I don't blow it
Sara: Relax. I'm sure you'll do fine.
Meaning: "to blow it" is used when someone makes a mistake that leads to failure.
http://idiomstudy.blogspot.com/
