What’s wrong with my leaf?

The dark leaf color and curling down on the leaf tip is a sign of mild nitrogen toxicity. I would give the plant a light flush of plain water then just keep using plain water on your watering schedule until you see the leaves lighten up some

When I was not paying how much I put and was just hoping that all I put will be consumed I knew, when I get that marble mosaic - I Put too much nitrogen and always slowed down for two-three waterings, but sometimes that mosaic was a sighn for heavy feeder and she was eating double the other plants would. And I grew 3-4 same strain, so I saw that marking. I think it's overfeed. All the Best !

If its just this one leaf it might be a random mutation and you can ignore it. Keep an eye on your plant and check for more of them. Maybe water was splashed on the leaf and it got burnt. Good Luck!