Not so long ago, Blackbeard showed off his ability to create sea quakes, when he went to Amazon Lilly. He can also spread darkness. The man with the burn scar can create whirlpools and his ship is cloaked in darkness. It would make sense if he was Rocks, because Blackbeard might have modeled himself after that man. He chose his two Devilfruits deliberately. Perhaps he wanted to be a stronger version of Xebec. Hawkins could do all kinds of things with the Strawfruit, that most people wouldn't think are related. Perhaps there is a Devilfruit with an ability set that includes darkness and whirlpools even so there doesn't seem to be a connection between those two powers.