Export a specific non-default database using drush sql-dump
According to the Drush 8 docs, there is a --database=key option that does what you want.

I've verified it against a D8 site with multiple databases using the command:

drush @dev sql-dump --database="blog_import" &gt; ~/tmp/blog_import.sql


The drush sql-cli command also has the same option so you should be able to import the dump using a command like:

drush @dev sql-cli --database="blog_import" &lt; ~/tmp/blog_import.sql