Unlike Windows Server, we don't restrict Mac OS X Server from being backed up to our servers, so you can run Backblaze. However, many of OS X Server's locations that it stores data in - /opt /usr /private and others like those - are not backed up to our servers. This would be important if you're using OS X Server features like web/database hosting, address book sharing, possibly other features like remote home folders may not be backed up or backed up accurately to Backblaze.
If your data is primarily in normal locations, like user folders, external drives, etc., it shouldn't pose an issue to back up OS X Server. However the bottom line is that we don't officially support it - so you'll need to keep that in mind.