I do not agree with it being the temp. I have a certain place that I grow where most of the plants itself will turn completely black/purple the leaves will also seem a bit dryer that usual, I do not know what is in the soil there, I do know that that soil grows the biggest plants I will have this year so it is not a problem, it is actually a godsend. This is the only spot that I have ever had this happen ever! I have grown multiple strains there and no what I plant there they turn out almost so purple they are black and they are f-ing giants.
If this thread last for a bit I can get pics in a day or two. One more note about this spot is sometimes the new growths get "bound up, on top of each other" anytime through there life there.