Although Soul Reap says it looks for “another black spell,” there’s no requirement that Soul Reap actually be cast (or be black) for this part of its ability to work. For example, if a spell such as Commandeer causes you to gain control of Soul Reap, it’ll still check whether you cast a black spell this turn, even though you didn’t cast Soul Reap itself.

A hybrid creature that’s green and another color is *not* a nongreen creature. Since it’s green, it can’t be nongreen!

Soul Reap doesn’t check whether you’ve cast another black spell until Soul Reap resolves. If you cast Soul Reap, then cast a black instant in response, the creature’s controller will lose the life when Soul Reap resolves.