I used a MinWax Dark Walnut Wood Finish Stain Marker. (also good for dark-stained furniture finish touchup!)
Dab it on and let it set for maybe 15 or 30 seconds, then wipe it off. If you leave it on too long it will become sticky and harder to wipe off.
Repeat a few times to get desired darkness. My sample pics, I used the marker 3 times.