I've read that if a requirement can't be verified by black-box testing or inspection, then it's not a requirement, but a design specification.
I'm building a software that makes processing with on images acquired by a camera, and the processing itself should be verified because it is essential for producing the right image presented to the end user.
However it is very difficult to test this in black-box, even in gray-box, so I wonder if it should be listed as a software requirement? If yes, then is it OK for my software verification to have it verified in white or gray-box testing?
PS: I'm building medical device software, thus following IEC 62304 standard, that specifies that my software requirements shall be verified through system testing.