In the grand scheme of things the character name of the Man in Black probably isn’t all that important — and actually this was reported, but never confirmed, back in June by E! Online’s Kristen dos Santos — but a listing in the LOST auction catalog reveals the name of the Man in Black (played by actor Titus Welliver). Click over the jump for the reveal.
The listing shows the actor’s name, the LOST logo and his character name: Samuel:
So there you have it, the two brothers were called Jacob and Samuel.