In reply to Leo.

As far as storing and retrieving data from postmeta goes, I think this could be handled quite nicely for folks transitioning their existing metaboxes to blocks.

Take this small example of a "Stars Block" that I put together just now (my first ever Guten-block!). It currently stores the star rating inline, in post_content, but that's really only a very small part of how the block works. So small, I didn't even need to define where it was being stored. If I did want to define where it was stored, the attribute definition can take an optional `source` parameter, which currently refers to how the attribute is being stored inside post_content, but this could potentially be expanded to allow meta fields to be defined.

I don't know if this is the best way, it will take some exploring to find the right way, but hooking postmeta data sources to blocks is certainly not an insurmountable problem. You can read some more discussion on the topic in this issue.