Were the 19 there to witness the man's execution and the lamp signalled that the electric chair had been turned on?
Still doesn't explain why they can't see. Maybe the electric chair overloaded the wiring and the lights blew out?
Hmmm... this idea reminds me of The Green Mile. Maybe it's the execution of John Coffey?

Nobody else is close to the solution of my riddle yet.

I would give more hints if Kweepa hadn't already proved it can be solved without hints. I will after a bit longer though.