I've used CastleSecretBlack before to hide areas of the Map from view. I've come to use this today, and got an odd issue that I was hoping someone could help with: Instead of hiding the area on the Local Map, the CSB is leaving the Area exposed, and superimposin a light-grey square around it (in the shape of the CSB)-- does anyone know why it might be doing this? It is definitely enabled, but just not working...

I just tested this in one of my test cells. If you put CastleSecretBlack under or on the floor, it won't conceal it properly. If you put it completely above the piece you want to conceal, it won't show up on the local map.

The Vyper is right; it needs to be above. The reason for this is when the game generates the local maps, it draws everything from above the cell. That's why you rarely go underneath an original path in a dungeon; and why the Root dungeons in Shivering Isles are so hard to navigate!
Another thing you can't do with them is disable/enable them in a script to reveal areas. The game only draws local maps on load up (if that often), and there's no way to refresh them that I know of.

Hope that helped.

Thanks for the suggestions/info, unfortunately, they're already above what I want to conceal The Room Objects are at 0 to +192 Z Positioning, with the CastleSecretBlack above them (tried different heights, even upto 60,000) but they're still showing-- the CSB is definitely there, and enabled-- if I disable it, then check the Cell again, the black square around on the map (which appears where the CSB is) disappears

Check a cell where they're already being used (Right Click > Use Info) and see what they're doing that you're not. Usually solves the problem.