Apple sets a hidden flag on some files indicating that those files should not be backed up. To tell Backblaze not to backup those files, select the checkbox labeled "Also exclude Apple-specified exclusions".
To find the setting, first, click the Backblaze flame icon in the menu bar, then select "Backblaze Preferences" from the menu which appears. In the "Backblaze Backup" preferences panel which appears, click "Settings" then select the "Exclusions" tab.
A checkbox for excluding Apple exclusions from Backblaze backups appears in the tab view, as shown below:
You can find which files will be excluded when the box is checked by opening the Terminal application and entering this:
sudo mdfind "com_apple_backup_excludeItem = 'com.apple.backupd'"
Please note that this command requires a system administrator password.