I have a black toenail, what could cause it and what could be done?

I have a black toenail. It just appeared a while ago I didn’t bang it or anything and it doesn’t hurt. I just noticed the black area seems to have spread a bit along the bottom of my nail. I obviously didn’t want to waste a doctors time as it not sore. Was wondering what could cause it and if anything i could do or buy to sort it out. I have included a photo of it.

Age: 41
Medications: Oxycontin, pregabalin, oramorph, amitriptyline, lidocaine patches
General Information: I have CRPS after sacral nerve injury from an operation that went very wrong. My left leg affected and currently immobile & housebound due to pain in my leg, hip and back.