Black is white, white is black by Engberg

As we can see while Sayid is being diagnosed, Dogen blows some ash on him. I believe this was to see whether Sayid, or Sayid's body, would react to the touch of ash. As we've seen around the cabin, and clearly seen in LA X when Bram circles ash around himself so the Smoke Monster can't get to him, the Smoke Monster doesn't feel too well about ash. Maybe the Smoke Monster would be exposed by doing this, or even drawn out of the body of Sayid.

This doesn't explain why Dogen said he didn't "pass the test," though, which we have yet to see explained.

So I believe that Sayid, or Sayid's body, may be possessed or have been taken over by the Smoke Monster, or another Smoke Monster entity on the island. And as suggested many places before, maybe Jacob, as a white Smoke Monster, or something else, maybe something of a lot of bright light, explaining what Locke saw back in season 1.

This, however, doesn't explain why Dogen suggested there was something dark inside him, something evil. So what if the whole concept of Lost, is actually white is bad, black is good? Meaning the other others being on Jacob's side, therefore they're bad.

Also, I think the Smoke Monster, or bodies being taken over by the Smoke Monster will have some sort of reaction to being burned, or exposed to high heat, explaining why Dogen burned Sayid's chest with hot metal.

So maybe MIB, presumably the Smoke Monster, will be affected when and if the volcano on the island erupts.