It is not just that there are blank spots on satellite maps out there that must terrify a geographer. It is that, after centuries of a Euro-declared Age of Exploration, there are concerted efforts to take away the precariously built-up bits of knowledge, the lovingly tabulated, mapped, and abstracted creases of mountain valleys and the twisty hidden jogging paths of the world's rivers. By taking away and editing from "the original negatives", you are not just obscuring a secret; to a geographer, it must be like the deletion of information.
A blank spot on a map used to mean, of course, something different - terra incognito. The presence of secrets indicated by a blank map suggests, instead, that everything that can be discovered, had already been discovered - which is precisely how the profession of geographer has changed from a cartographer-extra to the myriad specialties Paglen's book discusses. On a finite world, with all the information already uncovered, blank spots indicate spatial deletions. Blank spots somewhere else, however, suggest frontiers.
Blank within the discovered suggests obscurity and secrecy, something sinister that has been deliberately covered back up or furtively skirted around. Blank spots on a canvas of nothing suggests a horizon to be pierced. To map this cognitively, anything blank is a gap in knowledge, not so much a darkly held secret to be probed. To suggest that, despite how we might imagine the world as a geographical plane, there can be such a thing as infinite knowledge to be aware of the gaps within is, sadly, hubristic.
But which brings me, in a way, to conspiracies. Why would a non-geographer be interested in blank spots on a map? Helpfully, for those who professions aren't being purposefully destroyed, we can imagine something just as sinister. Secrets in themselves do not generate interest; the room you are in may be said to contain "secrets" from where the remote is (nobody knows!), to who slept with whom here 7 years ago (but OMG, don't tell). Secrets, like classified information, to be of any interest, must be usable knowledge and, furthermore, there must be effectual demand for that knowledge. Secret knowledge that we don't understand is of no practical reason. The secrets of fusion power generation at ITER make no sense to me, even if I were to get a copy of their readouts. Secrets gain their power - in the Foucauldian sense - only if they arouse curiosity.