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It may be because after the NVR's POST display, a resolution is attempted that the HDMI TV cannot display.
Can you hook up a VGA monitor to see if it correctly displayed there when the screen goes black on the HDMI TV? The specs say it can display to both at the same time.
If so, have you tried another HDMI cable?
Lastly, does the NVR have a configuration section to direct the output to HDMI and a place to set the resolution displayed there that works with your TV ?