This resembles a venomous bite of some sort. The venom would be injected into the area which is now darker than the rest.

I had a spider bite on vacation that looked like this. It started off as a little bump like a small mosquito bite and turned into a nightmare in 4 days. PAIN!

The venom remains in the dermis layer and destroys tissue and darkens with time. The redness around it all is the body reacting to the venom.

I went to the local doctor and he gave me 3 things for it.
1) Antibiotic --Zithromax incase of infection
2) Steroids-- high dose oral with a 7 day taper
3) strong antihistamine--Atarax 50mg.

The steroids did the best for me. But I had pain in that ankle for at least 3 months after. I don't have RSD ...so I used cold treatments on it for pain.

All spiders have some sort of venom. So any type can cause a nasty reaction. (the really venomous ones, we know about... brown recluse, black widow, etc). But even little ones not typically dangerous can leave a nasty reaction behind when they bite you. Keep the area clean to prevent infection.

My bite started to improve with 48 hrs on the steroids.