DIAGNOSIS: This is usually a handshake issue between the monitor's AMD FreeSync Premium and your graphics card when the game's refresh rate falls outside the supported variable refresh rate (VRR) range.
FIX: First, ensure FreeSync is actually enabled. Use the monitor's OSD joystick: go to Game > AMD FreeSync Premium and set it to On. Then, in your AMD Radeon Settings or Nvidia Control Panel (for G-Sync Compatible mode), make sure the variable refresh rate feature is also enabled for this display.
IF THAT DIDN'T WORK: The black screen flashes often happen if a game's cutscene or menu locks to a very low frame rate (like 24 fps) or if you alt-tab. Try limiting your game's minimum frame rate in your GPU control panel to something above 48 fps, which is typically the lower bound of the VRR range. Also, try using a DisplayPort cable instead of HDMI, as it can sometimes provide a more stable adaptive sync handshake.
HEADS UP: HDR (High Dynamic Range) and FreeSync can sometimes conflict. If you have HDR enabled in Windows or the game, try disabling it to see if the black screens stop. It's a known quirk with many monitors that support both features.