|1.||blah fuck it|
This phrase is used as a substitute for "ah fuck it", and really is just a creative use of the word "blah".
Specifically, it should be used when something doesn't need to be said and you realize it when you're halfway through explaining it or even in mid-sentence.
It could also be used when you suddenly change your mind about something.
I've got 15 minutes to get to this wedding and I can't find my tie!!
Blah fuck it, I'll go without one.