Various people, including me, have posted this before, but it bears repeating. A visual inspection cannot determine whether a tank is suitable for O2 service or even nitrox service, for at least two reasons.
First, not all combustible contaminants fluoresce under black light. Perhaps most importantly, synthetic compressor oils do not. Second, a black light cannot see around corners, and there are lots of corners in a valve. The cleanliness of the valve is arguably (much) more important than that of the tank, actually. Particle impingement during O2 fills has all those turns to make inside the valve, while particles that make it through the valve maze have lots of room to slow down before they get to the bottom of the tank.
Marking a tank O2 clean follows, well, O2 cleaning. Anyone else who accepts that inspector's sticker must trust the inspector or insist on a separate cleaning and VIS before restickering it.