A) a sound used to describe boredom, or mediocrtie; B)to find something wholly offensive as to want to throw up to purge the sensation from the mind and/or body; C) average
Rachel and Jonny are sitting on a couch. Rachel asks Jonny what he's feeling. Jonny replies, "Blech."
"I was driving and all of a sudden I started smelling something sort of like rotten milk and I knew Blech had to be near...she smells like rotten milk"