leaf color

Forgive me if this has been covered before. Please, could anyone help tell me what’s going on here (see pics)? About 4-5 days ago I noticed some of the leaves on one side had turned v dark (almost black). I initially thought it might be sun burn - Id left it out too long in the sun one...

I have a tree (~4m) that was dug up in spring and I guess barerooted and put into coco fiber/ gravel mix and then sent to me. I had no place to plant it yet so its still in that pot. Seems like an ok potting medium and I found some healthy roots when I dug into the pot. I threw some fertilizer...