DIAGNOSIS This is typically a handshake issue with AMD FreeSync. The monitor is momentarily losing sync with the graphics card when the frame rate fluctuates wildly or drops below the FreeSync range.
FIX Disable FreeSync on the monitor first. Press the menu button, go to 'Game' > 'FreeSync' and set it to 'Off'. Test to see if the blackouts stop. If they do, you can try re-enabling it with a more stable frame rate.
IF THAT DIDN'T WORK The issue could be a marginal HDMI cable. Swap in a certified High-Speed HDMI cable (look for the official certification logo on the packaging). Cheap cables can't handle the high bandwidth consistently.
HEADS UP If you're using an NVIDIA GPU with this AMD FreeSync monitor via HDMI, ensure 'G-Sync Compatible' mode is disabled in the NVIDIA Control Panel, as that can cause similar dropouts.