re: What can DNA prove in this case?

It can shed light on the whole story. For instance, hypothetically JJ could have appeared at the interview with cuts on his hand which he explained as the result of football. If it is then established that his blood is at the scene it calls his whole story into question.

On the other hand, it could b powerful evidence if none of the blood at the scene belongs to JJ.

remember, Stone said they are looking for evidence to prove his guilt or innocence.