The Lord Of The Rings: 10 Things You Didn't Know About The Nazgûl
6. The Nazgûl Were Blind
In Black Speech (the language of Mordor) Nazgûl just means Ringwraith. It's fairly obvious these are not normal creatures then. Although we know they were once men, nothing of their original bodies remain. They only have physical form when robed in the garments given to them by Sauron, otherwise only their presence would be noticed. They therefore do not interact with the world as others do.
They could sense shapes, but in day light their sight was very limited, and they relied on their horses for vision. They could only clearly see things that also existed in the Wraith World; Elf lords were visible to them, as well as Ring bearers, which is why they were drawn to Frodo when he used the One. But otherwise they found it hard to distinguish other forms and beings.
When the hobbits first encounter them, they note that they make a strong sniffing sound, Aragorn later reveals they have an extremely good sense of smell and use it as their primary way to detect foes.