I'll admit Oda hasn't done the best job of defining. And visually, the black shading idea is total crap.
But it's a cool way to illustrate a difference between the two halves of One Piece. The first half mostly sets us up to believe that first of all, DF powers are rare to the point of being seen as myths. Then we find that all the big players seem to have powers and that the fights will shake out like rock paper scissors.
Then at the end of the first half, we see that the rock paper scissors stuff is for scrubs. however common the powers really are, that the dominant force is really Haki. Guys like Roger may not even have had DF powers. But without Haki, ambition, you're not going near the top.