User blog comment:Am Shegar of the Fairy Tail/New skills/@comment-3290324-20150729205440/@comment-25324252-20150729211511

"Maybe its because he learned demon slayer magic too quickly. He let some of that evil stuff get inside of him" So its not cause he learned it too quickly that he has the black marks, its because he learned it too quickly, thus not letting him nullify Mard Geer's curse which he ended up absorbing part of.