An expression of sympathy. Not necessarily sad nor happy, it can describe self-pity or be a reply of empathy. It is ideally used when "yes" or "no" is not appropriate but yet the situation still requires some conclusion.
Self-pity: "Bleh" - I'm bored, tired and agitated but I'n not ready to shut-up and go home yet.
Reply: "Bleh" - I hear you and I feel for you, man!
In the uncomfortable quiet of a dying conversation, instead of farting, the word "bleh" just completes the situation.