How cold is it getting at night? I lost a Crown of Thorns once many years ago putting it next to a drafty window in the winter. Black leaves are usually cold damage. Remember that the bat flower is a true tropical and will be damaged at temperatures below 50 degrees, so if it's still getting cool at night, sitting next to that window, it may be getting too cold.

Also, it needs shade or indirect sun, like in an east or north window, not a south or west window, where it will get too much sun. I wish you could post a picture so we could see the leaves. The dark brown, leathery leaves could be sunburn.

They don't like wet feet. You need to repot it, since it is probably planted in the wrong soil and rootbound, as most plants are when you buy them. It likes rich, acid soil. I always potted mine in 1/2 peat moss, 1/2 general all-purpose bark-based orchid mix, and they did fine. Stick your finger down into the soil and don't water until it is dry up the second knuckle.

For humidity, set it on a large tray full of pebbles and keep water in the tray, but not touching the bottom of the pot. Or you can mist it, but without air circulation, that may just breed fungus. It needs good air circulation, so maybe get a small fan and let it circulate the air in the room.

Have you had the heat on, or is it sitting near an air conditioning duct?

How are you fertilizing it? I used to use a slow-release bloom fertilizer with monthly drenchings with liquid orchid fertilizer.

I hope this helps. I've only ever grown one outdoors in FL under a huge oak tree, where it was very happy, so I know nothing about growing them indoors.