A blah is a non-descript, boring, often stupid person that no one pays much attention to. Could be someone sitting right next to you at a restaurant, a bus stop, work, school, etc.
Often thought to be urban legends, but that's just because no one notices them.
1. Having a conversation with a blah like like talking to a rock. Except that the rock is actually kinda cool.
2. Some day the blahs are going to revolt, but nobody will care.
3. Have you ever seen a picture of a blah? Neither have I.
4. Huh, a blah sounds a lot like me...