Through speculation, I believe it to be both in a sense, that death WILL occur soon and someone WILL die.
But that isn't very true. Every time Magus mentions the black wind, no named chracter actually dies. Maybe it means that someone is
meant to die. For instance, he mentions it in his castle. Aside from the nameless mystics, the only candidates are the party, himself, and Lavos. Nobody dies. Magus likely did in the original timeline, but not thanks to Crono.
Secondly, he mentions that one among the party is going to die. Now Crono does die, but that's later undone by the Time Egg. So Janus's prophecy never technically came true.
The final one, if I recall correctly, is said to Schala when the party goes to her room. That one probably represents Schala and Queen Zeal's death in the Lavos timeline, but it could likely refer to the the entire Kingom of Zeal.
The Black Wind is definately an indicator sensed by few individuals which represents that someone will be leaving this mortal coil sooner than others may be.
By this do you mean that it swirls around those who die before they are supposed to, or those that die unnaturally? I can definitely see the latter applying.
Now since everyone in the world is "doomed" some day in their lives, since death IS inevitable, the Black Wind would not howl because the key moment for our lives to end has not come into picture yet. For us, the Black Wind might start howling on our hospital beds after a car accident or what not, for example. (no offense intended)
I'd think it would be a different case between dying on a hospital bed and being stabbed/blown up/disintegrated.