Yes, Backblaze will backup your email if it is locally stored on your computer. Email applications like Outlook, Outlook Express, Apple Mail, Thunderbird, Eudora, and Windows Live Mail will all have their email backed up.
Also, Backblaze will only backup the difference to the email database (PST, DBX, Database, etc.) and will NOT re-upload the entire email database.
However, Backblaze cannot backup files while they are in use, or otherwise locked by other applications. If you leave your mail application open constantly, you will want to make sure to close it at least daily to ensure the file is able to be backed up with regularity.