Well, that's my best guess based on a new rumor that has hit the net.
The rumor is stating that the actor cast as Black Manta for the Aquaman movie will share top billing with Jason Momoa and Amber Heard and be a seasoned movie veteran, which suggests a big-time actor (assumed to be African-American). It's also said Black Manta is being treated as a villain on the same level as Joker and Lex Luthor.
I can't really think of any other black actors that would better fit the bill better than Denzel Washington.
Morgan Freeman might be a bit too old at 79 years; Will Smith is playing Deadshot in Suicide Squad; Samuel L. Jackson is Marvel's Nick Fury; Laurence Fishburne is DC's Perry White, and Jamie Foxx just played Electro in the Amazing Spider-Man movies.
Michael K. Williams has been suggested to be playing Black Manta, but I think it's safe to say he's known more for TV than being a seasoned movie vet.
Denzel Washington would be pretty huge if WB could manage to grab him, and I also think he would be an awesome Black Manta. Some other contenders that come to mind include Eddie Murphy (in need of a revival) or even Ice Cube (recommended by Kevin Smith to play Green Lantern), and from Aquaman director James Wan's Furious 7: Tyrese Gibson and Ludacris.
We may find out just who is playing Black Manta in Aquaman as it's also said the casting news will be made this month.
The source for the rumor is via Twitter, so take this with a grain of salt.
Aquaman has an October 5, 2018 release.