What problem does this address?

After the development of Full Site Editing features and the theme blocks, there are a number of complex blocks that should be considered "power tools" for advanced users and should not be exposed by default to users unless they specifically search for them, such as the Query block.

What is your proposed solution?

A new block API should allow marking blocks as semi-hidden so that they only appear in the inserter as a search result when an advanced user is specifically searching for it.