I build a burner from a radar gun transformer that was surplus, and bought a pyrography pen at LeeValley, with a writing tip on it, for the reason that most pen inks will just disappear after a while , plus the problem of blotching with finishing.
Pyrography pen.jpg

I can write in all kinds of places and manners, even on darker wood you can still read it if angled to the light, though much easier on light colored wood of course as you can see here (older picture).

Bowl bottoms.jpg