* Collective nouns are words that refer to a group of things.
* "Rare black" typically refers to a collection of black things that are uncommon or valuable, like rare black diamonds or rare black orchids.
Since there's no specific word for this, you could use a general collective noun like:
* Collection of rare black diamonds
* Set of rare black orchids
* Assortment of rare black stamps
* Array of rare black pearls
You could also be more descriptive by using phrases like:
* A treasure trove of rare black objects
* A prized collection of rare black gems
* A remarkable assortment of rare black butterflies
The best choice will depend on the specific context and the type of "rare black" things being referred to.