Question

He made that poor piano moan with melody.
O Blues!
Swaying to and fro on his rickety stool
He played that sad raggy tune like a musical fool.
Sweet Blues!
Coming from a black man's soul.
O Blues!

The phrases “O Blues!” and “Sweet Blues!” are examples of
alliteration.
repetition.
hyperbole.
simile.

Answer 1

repetition.

The word/phrase "Blues!" (and the exclamation forms "O Blues!" / "Sweet Blues!") is repeated for emphasis, so it's an example of repetition.