Board Thread:Questions and Answers/@comment-24382143-20140109040537/@comment-3211055-20140110230000

Almost every Diabolus name (the part that written in Katakana) sans a few one is a Latin word with roughly the same meaning as the Kanji in their "Demon of X" title. You might want to check out all of them in online dictionary.

In this case, Niger comes form the Latin word for "black". Hence, the "Demon of Jet-Black" title.