I know about the DexNav and how 3 Stars means 3 IVs, but I always encounter Pokémon with 3 Black stars and some with 1 white star and 2 black ones. What does it mean? The black stars and the white stars?
If you run into a Pokemon with 3 Empty Stars (i.e. none of them are filled), this simply means that the Pokemon has no full IVs.
This is better than not seeing the potential rating at all, if you're specifically trying to get ones with IVs you can easily tell without starting a battle with it whether it'll be worth your time to catch it.
If a Pokemon has 1 star out of 3 filled, this just means that it has 1 max IV stat. The other 'unfilled' stars don't mean anything special in this case.