When Backblaze is configured as an additional destination, the destination never validates.
After BackBlaze is configured and you go to validate the destination, this spins with " Validating …" and eventually times out with the following error logged in /usr/local/cpanel/logs/error_log:
error: CODE :500: MSG :500 Timeout: Timeout [backup_destination_validate]
Verify that you do have a working account and also verify the server can reach the destination, as an example:
If you get a 401, you'll want to discuss this with Backblaze as they may be having temporary issues with their service. BackBlaze Support