Let's look at the following expression which you will come across several times in hollywood movies and novels.
"I / you blew it"
This expression means that you had a golden opportunity to do something. But instead you did something which caused that opportunity to get away from you.
Let's assume a situation here -
You worked in a company and you had a great opprotunity for growth, but it required you to move into some other city which you were reluctant to move to. So you turned down that offer. Now a few years later when you don't get much out of your career, (which you would have got if you had accepted that promotion offer, even if it meant to move to the other city.) you realise that you wasted a lifetime opportunity to make a successful career had you accepted that offer. Then you might say the following sentence -
I blew it or I blew my career. (i wasted such a great opporutnity to make a successful career)
Sometimes others are responsible for this.
For example - you and your colleague have put forward a business proposal to a potential investor. But your colleague says some unusual thing in the meeting which puts off that investor (who otherwise was getting to be interested and had almost given the green signal to your proposal) In this situation you might say the following to your other friend- (after the meeting is over of course)
"We almost had that deal pulled off but he blew it by making such unncessary comment."