Less reflected light, all else being equal, means the same amount of detected noise (whether using cameras or eyes), and therefore a worse signal to noise ratio

Less reflected light, all else being equal, means the same amount of detected noise (whether using cameras or eyes), and therefore a worse signal to noise ratio on the dark image. Worse signal to noise ratio means less information. Less information indicates worse detection accuracy. While there isn't evidence that dark-skinned people have harder-to-recognize faces, there is also no evidence to the contrary. It seems like the null hypothesis in this case, given no additional evidence, is to assume that darker images are indeed harder to recognize.