I'm not sure why that is black but the normal map looks perfect in the first picture. You don't need what is rendered in that window, so this is not a problem. Juat keep doing what you are doing because you're doing it fine
About the offset UVs... you don't need that now. It was just a tip. The texture tiles if you go out of the main UV box. One way to share texture space is put UVs on top of each other. That is ok if you are not generateing normal maps. But if you want to generate normal maps you will have problems if UVS are in the same space. So the other way to share space and not have problems with the normal map is to put the UVs in another tile. The normal map is generated just from the main tile, but when it is used in the material editor is taken from the entire UV space.