You create the private key on the client side so that you can encrypt it with a custom password before sending it to Backblaze. Doing this makes they key recoverable from you - as long as you remember the passphrase to decrypt it - and unusable from Backblaze.
> The priv key is NEVER written to disk on your system,it is then thrown away after transmission. The public key can be found on your local system here on a Windows PC (ask if you want to know on a Macintosh): C:\Program Files\Backblaze\userPub.pem This is a standard "OpenSSL PEM File" as decribed here: http://www.openssl.org/docs/crypto/pem.html#PEM_ENCRYPTION_F....
> You can read these files with Notepad or Wordpad, but be very very careful not to write out any changes or it will effectively destroy your backup -> make it unrecoverable/unreadable.
Their client is not designed this way. Their installer generates and sends the private key before the user has an option to set a passphrase. I replied to a different comment about this with more detail[0].
Also, the encryption happens on the server. If you protected the private key with a passphrase from the moment it was created then your data couldn't be encrypted because backblaze would never have access to the key.