My Vega 64 is a sapphire reference card with a aio water cooler on it. Recently my card has been crashing when playing games. It never reaches more than 70c. Whenever it does crash I have to restart the PC even though the other components are working fine. It crashes to a black screen and sometimes there's one green light on the tachometer.
Try reinstalling your graphic drivers with a factory reset in options at the beginning and the option to delete saved precepts. This has always, and I mean always, fixed all black screen issues for me.