Dwarves and the northern war of the rings

[Manwe Sulimo]

It was a nazgul, it could only be one from the discription

You mean from the "black rider" quote? Remember, the Nazgûl weren't actually "black riders"; they were "disguised as riders in black" specifically for their trip to the Shire. Who says that the messenger couldn't have been the Mouth of Sauron or some other lackey?

Brand probably kept it secret, to mull over and figure what it meant about the future. Other human leaders have been known to do that (*cough* Denethor *cough*).