'Blah' is a replacement word for 'bro' or 'brother'. It is often used when the speaker is not bothered to pronounce the listeners name. 'Blah' is often used after the word 'chyeah' to emphasis excitement or reinforce the speaker and listeners understanding.
Speaker 1: "hey dizzle, you enjoy the party last night"
Speaker 2: "yer it was pretty sweet, did you end up taking the chick back to yours?"
Speaker 1: "chyeah blah"