python - detecting when the camera view is blocked (black frame)

I'm trying to detect camera tampering (lens being blocked, resulting in a black frame). The approach I have taken so far is to apply background subtraction and then finding contours post thresholding the foreground mask. Next, I find the area of each contour and if the contour area is higher than a threshold value (say, larger than 3/4th of the camera frame area), then the camera is said to be tampered.
Although upon trying this approach, there is a false tamper alert, even when the camera captures full view.
Not sure, how to go about this detection.
Any help shall be highly appreciated