Exporting block fields

The hook you should be overriding is hook_content_export_entity_alter, by replacing "hook" to the name of the module that is implementing the hook, for example for the single content sync module, it would be single_content_sync_content_export_entity_alter. To implement the hook, the resulting function should be placed inside the .module file.

When you look at the hook in the single content sync module, you will see a switch that has a single case of 'my_custom_entity'. In your implementation of the hook that should be replaced with 'block_field', plus whatever else you need to alter to perform your export successfully, such as adding fields that should be exported from that content.

Thank you, this is the info I needed!