SQL Server logs "blob data", viewable with journalctl
This happens when there are unprintable characters in the output. 

It seems sqlservr is outputting \r (or ^M) (line feed) characters. 

You can view the raw data using: journalctl -u mssql-server.service --all

From what I can see, this appears to be the same content as /var/opt/mssql/log/errorlog, so that might be an easier way to view logs.