Earl, I wholeheartedly agree that blues can be quite joyful. It can run the whole gamut of emotions. Check out Big Bill Broonzy's "Long Tall Mama" for one of the most joyful as well as technically difficult rag-style blues guitar pieces.

Robert Johnson - I think his two best were "Kind Hearted Mama" (especially the take with the guitar solo midway through) and "Love In Vain." The second is a fine example of minimalism in the blues - it's as important for what it leaves unsaid and unplayed, as for what is said and played.

Blind Willie McTell was a prolific and prodigious talent, largely unappreciated in his own times. One of his most obscure but I feel one of his greatest songs was "Drive Away Blues." His use of repitition in the lyrics reinforces the mood of the song, which is that he is drunk, confused, lonely, and losing his mind - "You drove me away and you drove my heart insane."

An urban blues with the same theme is Albert Collins' "My Mind is Trying to Leave Me." I'd have to put that one on the list too.

I could go on and on but I'll let someone else have a turn.