Blehck is a way of saying you feel sick or uneasy. Or it can be a way of saying that an object or an article of clothing or a person is just so unpleasing that you can't think of a good enough word to name it.
Example 1. Daniel "sorry man, I can't see the movie tomorrow" Reid "why not?" Daniel "because I'm feeling just sorta Blehck right now.
Example 2. Daniel "did you see Jesse's face the other day?" Reid "Oh yeah dude! it was all....BLEHCK!"