I might remember wrong but i think i saw yami-yami as a paramecia.
And here's my point how it has a great chance to be so:
In the Impel Down Arc Luffy has a quick fight with Blackbeard. At that time Luffy didnt know armanment haki so if Blackbeard was a logia Luffy shouldn't be able to punch him.Luffy used conquerers haki in Amazon Lily but when you have conquerers haki doesnt mean you will have armament haki. And as Blackbeard said when fighting Ace"I wont get damaged even if i do my darkness will absorb it." or something like that. I thought the paramecia fruit "yami-yami no mi" had a strange healing power like Marco's pheonix fruit. And against my point i have this: darkness is probably an element and logia enables you to become and control an element. If this is true then in the fight in Impel Down Blackbeard didn't want to become darkness or Luffy was super fast for him then which the 2nd one is more believeable.
I want to know your guys' opinion