Single plant leaf finger has turned purple/almost ...

Congrats on a very cool and very rare genetic mutation. This could be caused by the start of senescense , the begining of the end of the plant's life - usually you'll get "autumn colors" not straight up black though! Anyway as in all thing genetic, you can't do anything about it, and you can expect it to get "worse" (better? More awesome?) As you flush and induce full on senescence. So for the flush the main idea is to get rid of excess nutrients. So as you you know there are three categories of plant juice : nutrient, micro nutrient and additives. For the flush you can use additives , as I do, to flush more effectively. So additives like enzymes , chelation agents like amino acids (humic and fluvic acids) or hydrochloric acid (no joke, this is often used!) Also known as flushing agents do help break down the nutrients and get rid of any excess nutrients. So the baseline is 3 weeks plain water flush in soil for the best smoo hear smoke or with additives like enzymes that's 2 weeks, with amino acids that's 10-14 days, with flushing agent maybe just one week 10 days will do. But you will be good with plain water. Try using ice water or cold water the last two weeks , it will do something with her roots and help get those colors and terpenes in. Anyway man hope this helps ! Congrats on a fantastic first grow ! 🚀