Samuel was the name used by casting agents when filling the role of the Man in Black. It is not, however, the actual name of the Man in Black character. Below is an e-mail from Gregg Nations to Lostpedia regarding this matter.
On Aug 3, 2010, at 2:29 PM, Greggory Nations wrote:
Don't know if you've seen the posting on DarkUFO, but as part of the auction, they are offering the chairback used for Titus Welliver on the set. On the chairback is what fans are thinking is the Man in Black's character name: "Samuel."
That is wrong and was never intended to be his name. Here's a little clarification on the name.
The Man in Black was always referred to in the scripts as the "Man in Black." Damon and Carlton had to cast an actor to play the role, and when preparing the casting breakdown, they knew they had to include a name. They couldn't call him "Man in Black" because that would be a big red flag for fans and would reveal too much too early. So they told me to use the name "Samuel" since it was only for the casting breakdown.
Who knows why production used "Samuel" as opposed to "Man in Black" for the chairback. Titus didn't know his character's name since he never was named, and he may have told someone in production the name used for the casting breakdown and sides. (The breakdown also listed him as a corporate raider, and that obviously isn't what his true job was.) But it's just a placeholder name and was never meant to his real name.
If you could pass this along to everyone, I would appreciate it. I don't want fans reading into it when there's really nothing there. Thank you!