Hard to know how you're watering (you didn't explain), but blackness usually means rotting somewhere, which usually means way too much water or a fungus that existed previously. Take it out of the pot and cut away any roots that are mushy. Repot into a fast draining with lots of grit (perlite, small aquarium gravel) in it - no shards or anything on the bottom though - and water only when a lot of the mix has dried out unless leaves actually droop (after a drying out time). After repotting, use a house plant fungicide with the first watering. Keep it in the most sun you have, don't let the pot ever sit in drain water, but do keep a wide saucer of stones and water underneath, with water not reaching the pot, as it does need humidity (and spraying doesn't work!).