Replied by Ted (Bangkok, Thailand)
It's possible that the black streak came from a fungus (sometimes a bacteria but not often) that is actually black and hides in the cavities or crevices of the teeth that toothbrushes can't reach.
One way to remove the blackness, is to oxidized them using a 1% Hydrogen Peroxide as a mouthwash several times, then do the oil pulling. The black streaks should no longer exist.
