That's very strange Becca. Is there any possibility it dried out completely? I've seen black streaks on severely underwatered plants particularly on new growth. Coulds water from something else have dripped on it from above? Is the black spreading?
Terri, I have been keeping it pretty moist. I tried to pull the rotten leaf out as suggested earlier....it is inside of the leaf that is showing the brown and causing the problem with the other leaf....it was crispy dry! The other leaf on the outside shown in the picture isn't crispy brown, just soft brown and won't come out with a gentle tug.