The doctor didn't seem to suspect MRSA, he said it was most likely an insect bite that got infected. I told him I thought it was a spider bite and he didn't really comment on that.
The necrotic part that I'm describing is the black tissue and cavity under the surface of the skin. The first bite that I had years ago turned black under my skin in an area bout the circumference of a nickel, then I was left with a cavity that could fit the tip of my finger, but this happened under the surface of my skin and you could only tell the cavity was there by the blackness that lingered for a long time and if you felt it with your finger (sorry, gross!), my skin never broke open like it did this time. This time it was significantly smaller but looked about the same in that the tissue under my wound turned purple/black like I had a really deep bruise, but that went away quickly. I'm looking at it now and it still looks gross, but it's an open crater with a big scab inside and no more black stuff (I have no idea what happened to the black tissue because it was never exposed, it just kind of went away before the blister formed) and there is a sort of hole in the center of the scab. It's not weeping or anything like that, seems to be healing nicely and the edges of the crater are still a little red but not like before and there's no more swelling. I heard about the covid toes thing but I'm not reading about it because it will probably just freak me out, but from the little I've managed to pick up about it the black patches from Covid seem to be happening more on fingers and toes due to a circulation problem, this is on the underside of my forearm, by my elbow, exactly where I might have leaned on a spider or some other bug accidentally. I can't explain why i never felt anything though.
I actually googled what a MRSA rash looks like (and I'm kind of sorry that it did because it's awful) and it doesn't look like what is on my arm and like you said, my kids and I have all been home now with only each other for a month and a half so it's extremely unlikely that I would have been exposed to that. There is a possiblility of me being exposed to other people's germs because we live in a townhouse and we share an outer door with our neighbors but I always wash/sanitize my hands after touching the shared outer door so that is unlikely as well.