I, recently, bought a mix of flashbulbs.
There are some press 25, 40, 50 and GE equivalent.
Some are transparent and some are blue, but there are 1 for type that are black (or very dark brown...), and I don't know for what task are they so.

What I' sure is that aren't used ones...

bye

Massimo

Clear are called tungsten blue ones are color corrected to be close to daylight the black or dark brown maybe the blue coding is just discolored

I'm guessing the black ones are infrared. GE for example made type #5R which have a coating that just allows the IR light to go through, giving you an "invisible" flash.