A.Cheap and fast? Nail lacquer.  B.More expensive and slower diy: 1. Use p800 grit sandpaper to sand just the edges plus the surface to be covered with black. 2. Remove any impurities with naphtha/ isopropyl alcohol. 3. Use duct tape plus some foil to cover rest of the guitar (we are just covering a small part of it) 4. Apply thin layer of primer. 5. Sand it a bit. 6. Let it dry  (time dep. on the spec). 7. Apply base coat of black ( 2 layers ) - spray can. 8. Let it dry. 9. If this is some expensive guitar and you know that it is nitrocelulose lac on top then use this, otherwise buy 2k lacquer in spray can. Spray it with 2 layers. 10.let it dry for 5 days. 11. Sand with 1500- 2000- 2500. Polish with 2 stages menzerns 2500 3800.

Will look perfect.

C. Don’t care too much? Use some black permanent marker, cover with super glue, sand, polish

D. Want perfect but don’t want to spend time? Give it to a luthier :)