Your tree has insects. Probably aphids, mealy bugs or scale. The black "stuff" is call sooty mold. It is a growth the developes on the sticky excreations from the insects. Check the branches and leaves (both top & bottom) and you will find the insects. You can either wash the tree using a nozzell on the end of your hose, or in the bathtub, or apply a horticultural oil such as Volk oil or Ultrafine. Millet

My greenhouse trees are also having this problem. It just started in the last couple of weeks after I enclosed the greenhouse for the winter months. I have been using a hard water spray from the "flat" setting on the hose nozzle with mixed results. I get one are cleared and the next day it is on new areas of the plants. I have Lilly Miller 'highly refined horticultural oil'. Would this be the equivalent of the 2 that Millet cited? I have noticed a couple of references, in this forum, about not using oil at this time of year. Why would that be a problem? Does this concern also extend to greenhouse plants?

Not using horticultural oils at this time of the year pertains to citrus trees growing OUTSIDE in the open weather. The reason is that horticultural oils applied now can reduce the tree's ability from becoming hardened off against a cold spell. However, this, of course, does not pertain to trees growing indoors or in greenhouses, as they are never confronted by cold weather. I do not know about lily Miller horticlture oil, but I would assume it would be comparable to Volk or Ultrafine. - Millet

Twisted leaves sounds a lot like leaf minor but I would be surprised to hear that they are in Z6.

Ned

Ned, the problem is, as far as I know, Kyartist purchased his citrus from nurseries in Fl.
I too thought about LM's, and it's possible the citrus carried these pests on their journey from point A to point B. Toni