Leaves Have Turned Black.... but only 2 leaves affected????

I have looked at quite a few threads and have still not found this exact problem... i have four plants, under 18/6 400w but have a good foot and a half of space between them... i noticed the almost dieing looking leaves a few days before i repotted into their final pot an 11 L. The leaves are exclusively on the bottom leaves of my cuttings and only one leaf on each.They have turned into a dark black from the tips in but are getting worse, the leaves are limp and withering. I re-potted into coco plus.. and have started with some nuts... coco a and b, but i have only started that in the last day and the leaves started off a good few days ago. What could be wrong.. I will post some photos... Any help is greatly appreciated cheers

The pictures aren't so good sorry.

Think it was too much humidity in the end.. pictures don't really show the problem. however they have started with a little bit of nute burn after i started using coco a and b with canna coco plus. should i cut the affected leave off?