The Black Wind is a negative, pervasive aura that arises when someone is near death or a catastrophic event is about to occur. Schala and Magus can acutely sense it; both knew something destructive was about to occur while in Zeal, and Magus felt it a few times further while traveling with Crono.
The Black Wind is an elite group operating within the Porre military, which uses it for spying or combat in important situations. Little is known of its size or full spectrum of duty, other than that gifted and dignified soldiers are often invited to serve. Norris is probably the youngest leader, having risen through the ranks of the military sharply and illustriously. The origin of the unit's name is similarly a mystery; one theory postulates that perhaps Magus's speeches to the Mystics mentioned a fearsome black wind, and the Porre unit took this name in order to exude intimidation. Since the Black Wind was a popular concept in Zeal, Dalton may have borrowed it when creating this unit. On an unrelated note, Direa feels the literal Black Wind around Serge, proving others can feel it:
Direa: ...You possess a young but gentle air about you... Hmm? There is an odd wind that is whirling about. I have heard that such wind- bearers appear at times, but... Direa: ...Do you speak the truth? That you are already dead in this world...? Direa: Hmm, I see... So that's why you have an odd air about you. As for a way to return to your original world, I do not know. But there are always reasons why such phenomena occur...
From the Arabic 'huayra yana', which is the wind of ashes that comes when someone is dying. At least, that's the theory, as it seems impossible to find any kind of information on this phrase. It's likely whoever put this forward made it up.