Originally Posted by Maksutov

This is the same reason why the bright umbras of sunspots appear black when contrasted with the brighter photosphere. It all has to do with the effects and limits of contrast that are inherent in photographic (and other visual) media and the human eye, and how the eye interprets such contrasts.

BTW, let's take your approach to sunspot umbras emitting no visible light because they look black and apply it to the above photos (and many other Apollo surface photos). There are no stars in the lunar sky. The sky in the locations where the stars should be is black. Therefore, per your interpretation, the stars in the photos are emitting no visible light. Of course, that is incorrect, as is your interpretation of pictures of sunspots.