* A filler word: Used when you can't think of the right word or phrase to say, or when you want to avoid saying something specific.
* An expression of boredom or disinterest: "That movie was just blah."
* A sound of non-specific speech: "He just sat there saying 'blah, blah, blah'."
It's unlikely someone would choose "Blah" as a name, unless they were deliberately trying to be ironic or humorous.