If you use a white eraser, you can gently (in circular motion) remove/lighten the stain. That's what I did (newbie nubuck owner) then I sprayed a suede/nubuck protector on my bag and it has been good thus far! Just make sure you hold the spray about 1 foot away from the bag and spray swiftly, don't saturate one area that can cause the area to become darker. The bottle I use recommends that you spray the bag 2-3 times to be able to cover all areas and make sure to let the bag dry for 24 hours before spraying again. It's a long process, but well worth it if you plan to use the bag a lot.
Thanks a lot for the info! Will try the eraser. I tried the rubber part of the brush for nubuk and it didn't do anything.