Grapefruit tree

Question

what causes the leaves to turn black

Answer 1

This is likely Sooty Mold, which is caused by Aphids or Scale insects.
These articles will help you.

http://ucce.ucdavis.edu/files/datastore/530-15.pdf
https://www.gardeningknowhow.com/plant-problems/disease/how-to-get-rid-of-sooty-mold.htm