In reply to David Thrift.

I'm not sure how this applies to WordPress.com rather than Gutenberg as a whole?

WordPress.com manages reusable blocks in the same way that core does, simply go to the block inserter, scroll down to Reusable Blocks, then click on 'Manage All Reusable Blocks'.

The Manage All Reusable Blocks link is also available from the editor settings icon too - '...'

Hope this helps!