Assassin’s Creed IV: Black Flag already has 59,000 pieces of (mostly) pointless DLC, so it’s not terribly surprising to learn that more of it is going to be released in the near future. The trophy-tracking site PS3trophies.org lists four achievements relating to an add-on called “Blackbeard’s Wrath” which looks like it’ll be multiplayer specific.
Three multipayer characters are mentioned in the achievements; The Jaguar (not a literal jaguar, probably,) The Orchid and Blackbeard himself. There’s also a fourth achievement which doesn’t mention any characters, but does talk about putting booby traps on lifts.
This information comes from a PS3 source, but the same DLC would no doubt be coming to the PC as well.
The already-announced ‘Season Pass’ for Assassin’s Creed IV says it includes additional multiplayer characters as part of the deal, so Blackbeard and company will probably be released as part of that. The first add-on from that pre-paid bundle, ‘Freedom Cry’ hasn’t been released yet (and follows the adventures of Adéwalé, your Quartermaster pal from the main game.)