If you see the result image, its not exactly compleyely dark. If you subtract the similar pixel elements, definitely you get the black pixels (zero). I don't seems there is any issue with result, as most part of the both suntract images are same. There are minor differences in both images, which are reflected in result image (see some non black pixels).
One Suggestion: If the images same type, make it double then do the subtraction.
What is your aim to doing so?
