Black items are rare in the game and it is hard to create an attractive decoration using them.
On the occasion of Halloween, I made such a carpet, we called it "Dementor Carpet" and as you can see without light effects it's hard to have a black effect, while with the light effects option turned on it looks dark, and is black enough to match various spooky decorations:

It seems to me that it is with the use of the "light effects" option that you can get the most desirable effect.
To get the most black SQM possible, you can use items such as:
Ghazbaran Carpet 
Rift Carpet 
Ballistic Boulder 
Yielocks 
The Supreme Cube 
Pirate Tapestry 
Black Pearls 
Coals 
Onyx Chip 