When you were cleaning it.. did it feel rough like a scab or smooth like maybe just some discoloration? Did you get any dried blood off or did it stay kinda the same? To be clear though.. you're asking about the dark spot, not the light spot, is that correct?
Sometimes it's easiest to work with a skittish bird.. after they've gone to roost at night.. using a flashlight and keeping it dark to get them off roost prevents chasing. Hold wings down to the body while apprehending and handling to prevent the panic flapping.. and proceed as needed.
The feathers look a little rough or thin right above that area.. have you seen her scratching at it at all?
So it does NOT sound immediately life threatening.. and possibly not even concerning.. It almost looks a *bit* like a tumor which could go either way long term.. but I mean it could be an irritated mosquito bite, etc.. I don't have enough information to say for sure.. she looks like a red/gold sex link type??.. they're (allegedly) known for their reproductive tumors and short life spans.. I try not to worry about that or speak it into existence but still paying attention.
Regarding the mites question.. do your after dark check and look at the skin below the vent and on the abdomen. Spread the feathers apart and look for anything running away from the light.. is her skin light and normal pinkish like your skin or is it angry, red, rough, or scabby at all? Any "dried dirt clumps" (lice eggs) hanging on the feather shaft near the skin?
What is her age? Do you know her place in the pecking order, or have you seen any reason to suspect issues.. Seem more like if there is any pecking it would be at an accidental wound and not actually cause by the flock.. but could definitely escalate into a flock problem.. Usually not a problem for MY flock. but dynamics vary widely  and it's seen commonly.
I hope she stays well and continues to thrive! 
