What can this be?

It's in the middle of my upper back. It started off a few months ago as just a little bump/pimple, then yesterday my husband saw it and was shocked at how big and dark it got. It sometimes itches me but other than that I don't even feel it and since it's on my back I don't notice it. I'll make an appointment in the morning to see my PCP, but until then does anyone know what it *might* be?

Thanks in advance for any insight.