Is there any "block_user" module?
There is no block_user module so far as I can see or find. 

You can delete the row from the key value table, yes. It should get rid of the error. You can do it with drush if you want.


Back up your database
drush sql-query "DELETE FROM key_value WHERE collection='system.schema' AND name='block_user';"


This assumes a Drupal 8 site.