Most commonly used in the same context as "oh, damn". Used to express frustration, anxiety, depression, etc. Ocasionally the second syllable is prolonged to demonstrate any emotion in an extreme fashion.
As an exclamation on it's own: Instead of "I can't believe I'm so stupid!", just use "Blerg!"
A word... or sound to describe how you're feeling. Usually representing an indefferience or simply not caring. Also used to exress boredom and uninterested in doing anything.
Sallie: "Blerg"
Sam: "Yea i'm not feelin so great too. Im SO BORED!! Blerg =P"
Example: You got a "C" on your History exam. In disgust you mutter "Oh, blerg!". Originally used in place of a cuss word in the old (circa mid '60's) "Archie And Jughead" comic books.