[Globu]

I don't know if this will help, but one little trick I found very helpful for smudging or smearing or any sort of thing like that is to use the hand-drawing select tool (I haven't reinstalled it since the HD crash, so I can't remember the exact term it used) to carefully select only the area you want to smudge/burn/whatever. That way, you can't push color into the surrounding pixels even if you try.

(Another trick in a situation where you only want to avoid tainting the black background is to use the magic wand to select the black background color with 0 tolerance, then Selection->Invert.

Don't know if those tips are helpful, but thought I'd throw 'em out there anyway.