Blocked numbers are synced through iCloud and push over the air to other devices that are logged into your iCloud account when Contact syncing is enabled. Even if you set up a new device from scratch, (rather than restoring a backup) they will show up after you log into iCloud and toggle Contacts on.
You can view your iCloud contacts on a PC by logging into iCloud on a browser on your PC and you can export them from there. I don't think it includes a list of the blocked numbers, though. I'm pretty sure it's just the contacts.