What's wrong with my plant? black coloration

Lately I’ve been noticing some black coloration on some leags, should I be concerned. If so what should I do? Indoor/dirt medium.

just of a pic like that i would say phosphorus or maybe magnesium issue, but that would be a wild guess fill this out maybe we can help you some more

Answer these simple questions the best you can.
If you do not know, or do not use something; Just say so = NA
Strain; Type, Bag seed, or NA
Soil in pots, Hydroponic, or Coco?
System type?
PH of runoff or solution in reservoir?
What is strength of nutrient mix? EC, or TDS
Indoor or Outdoor
Light system, size?
Temps; Day, Night
Humidity; Day, Night
Ventilation system; Yes, No, Size
AC, Humidifier, De-humidifier,
Co2; Yes, No
Add anything else you feel would help us give you a most informed answer. Feel free to elaborate, but short, to the point questions and facts will help us help you