Bleh is synonym of Blah. Bleh is the word you say when you're frustrated, or when you feel like puking. Or just when you're bored.
Some people use it as an adjective, like in "I feel so bleh"... It this case, it means that the person is in a mood to say "bleh".
-you're so bleh
-I'm gonna bleh
-Bleh, bleh, why me?