The ultimate fate of the ringbearer is to become a wraith. We are told that the wraiths don't see well in the light, but can see just fine on their "world"--the side to which Frodo is being drawn, where he almost found himself due to his wound from the Morgul knife. I wonder if what happens here is that Frodo, under the influence of the Ring and in such close proximity to Nazgul HQ, has his visual receptors  momentarily switched to the other side, enabling him to see Minas Morgul in greater depth, but unable to see clearly in the "real" world. Maybe a part of Frodo's ongoing struggle is to keep his mind out of the wraith world, a task requiring all his powers of concentration. So what happened the first time he "went blind" was that the unexpected call of the wraith, coming while he was preoccupied with climbing, threw him unprepared onto the other side. That might account for the effect of the Elven rope pulling his mind back from the unwholesome realm. Here in Minas Morgul, he is used to the struggle, but its sudden intensity momentarily overcomes him. Sam's function is to again root him in the living world, while Gollum's is to remind him to be vigilant.

I guess all that qualifies as a wildly UUT.

Outside of a dog, a book is man's best friend. Inside of a dog it's too dark to read.
Marx