I have a Black Cayenne that is growing great but some of the leaves have a black tint to them. They are not dieing out and the plant is putting on peppers. Just don't know where the tint is coming from. The plant next to it has not tint to its leaves
really strong light can cause leaves (and peppers) to darken, it's a defence mechanism for the plant. even though the other is not doing it, it may just be that one plant is "strong" enough to tolerate it and the other isn't.
another possibility is that a genetic trait for dark or purple foliage and/or stems is present in one of the plants. and since "black" is in the name of the plant, this is entirely plausible.