Hard to say, black on leaves usually means rot. Could be that they are to close to the medium and getting some splash back.
This happens a lot in my garden and I trim the lower leaves to close to the soil outside.
I would pluck them off off. and try and fold the others away from the medium or also pluck them.
Try not to over water as this can exacerbate the situation.
Best Of Luck!
Personally, I wouldn't worry about it! And I'd probably trim them off ESPECIALLY since they're on the bottom! If you see the same thing happening higher up, then you know you have a problem but for now - meh! 😊