A custom block is made of two entities, one for the placement and one for the actual content. Only the actual placement can be exported with cim. The content can not. Therefore this will result in "Block description Broken/Missing" error on site where the config is imported. And since there is no option to disable custom blocks from being exported through Configuration management, this will break the functionality.
Steps to reproduce
On Site A:
Create custom block
Assign this custom block to any region
Export configuration of the site
On Site B:
Import configuration from site A…