I think it's pretty clear that the Nazgul were permanently invisible, including their "original" clothing/armor etc (which Frodo could perceive on Weathertop and at the Ford); the black cloaks (donned of course after "fading" and Sauron's reclamation of their Rings) gave visibility to their forms which were unseen but not incorporeal.
(In an early, rejected draft for the scene at Maggot's house, T appears to be saying that if one put on the Ring and then picked up an object, that object would remain visible since it wasn't in the ringbearer's possession when he flipped the "engage invisibility" switch.)
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The entire plot of The Lord of the Rings could be said to turn on what Sauron didn’t know, and when he didn’t know it.
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