I have just switched on a small lamp which failed to work. I assume the bulb has gone but when I removed it instead of rattling it is just completely black.
I've never seen this happen before....is it just a normal thing for a bulb to do rather than ping and go out when switched on?
If it isn't a UV bulb and wasn't previously black I think that air has got into the bulb and oxidised the filament when it was switched on. The black coating on the inside would possibly be blue tungsten oxide.