Michael Ealy really would make a fantastic Black Manta. He has a darkness looming in him that would do Manta justice. His role in "For Colored Girls" was his darkest yet and seemed like a completely different person from how Ealy usually is.
Here's a clip that showcases how dark he can get, but be warned, it is not for the faint of heart. It's actually a pretty hard scene to watch and just gets even harder to watch towards the end.