I know you suffer with humidity in Texas just like we do here, but you do have more dry weather than we get in Florida, too. Lindsey lives in Ohio, so can't really plant a citrus tree outside. But I agree, it would be doing better if it could be out in the sun for at least the summer.
That black sooty mold is not caused by water on the leaves. If that were the case, every citrus tree in Florida would be black for most of the summer - it rains here sometimes every day for weeks. The sooty mold is caused by an insect infestation like aphids. They drop honeydew that stays on the leaf surface and the black sooty mold grows on that sticky, sweet stuff.
Lindsey, I'm wondering about the temperature in your room. You're probably running air conditioning right now, but if you're out during the day, any chance you can close your a/c vent to let it warm up in your room a bit? These trees do their major growing in the summertime when it's over 90deg. most days. Maybe your little tree thinks it's winter?
