I believe that Blackbeard can transform his body into darkness using his DF powers.
Allow Me To Explain:
Despite it not happening in the show (not YET anyway), the Yami Yami no Mi is still a Logia fruit and should be capable of transforming its eater into its respective element.
Blackbeard said that he can't DODGE things by changing into darkness, because darkness pulls things into itself instead of letting it pass through. And lets face it, Blackbeard isn't that great in the ol' reflex department.
In comparison, the Numa Numa no Mi is very similar. When its eater transforms into mud, projectiles don't pass through it, but rather, it gets absorbed into the eater's body.
If Blackbeard did change into his element, he would merely absorb everything he touched like the typical darkness he produces.
Haki could still hurt him, with the additional damage penalty just like when his normal body is hit.
I even came up with a cool name for the transformation: Black Beast
What do you think?