I've had black screen several times, each time I try an OS uograde. Need to remember to disvonnect all peripherals next time to see if it helps. All nvram anf reset doesn't work and there is no startup chime. It seems rather random what "fixes" it but the last thing i had been doing before it suddenly seemed to start properly again is I held the option-cmd-esc for an extended period. Hope that helps someone.