It is a termed used as a substitution for saying anything that you are to nervous or afraid to say.
"How do you feel about Ronald" Jessica said
"blahhhh." Pete said
"blah?" Jessica said confused
When a decently attractive thot sends progressive thotty pic and u go "BLAHHHH"
Blahhhh- Thot *sends thotty pic*
when trying to finesse hoe for the sexually arousing photographs