If it was a bloodstain it should have changed color to a rusty brown by now. Budgies don't produce red pigment so red isn't a natural feather color for them. It would REALLY help if you could post a picture. I know she isn't going to pose for an under-wing picture, but if you could get an adult or two to help hold her you could gently stretch the wing out and inspect it.
If she has some kind of dirt or minor irritation under the wing, giving her a mist bath might solve the problem. You get a spray bottle and set it on mist, then squirt the water up above her so it falls down like rain. With untame birds it usually works best to do this in the cage (take out the paper and the birdseed first), and if you do it in a room with a tile or linoleum floor (like the kitchen or bathroom) it will be easy to wipe up the water afterward. If your birds haven't had a bath like this before they might be unhappy about it, but birds naturally bathe in the rain so they should eventually come around and start spreading their wings and fanning their tails to catch the drops.
Like this:

Some birds like to bathe in a bowl of water:

This is a pineapple green cheek conure that I babysat for a couple of months. He was really funny when he took a bath, like he thought he was a fish.
