Backing up your Blockchain.com Wallet is critical and can be done by following this guide. Writing down the backup phrase will secure all of your cryptocurrency balances.
Because your USD-D address is the same as your ethereum address, the process of exporting the private key is the same. You can read about how to export the private key of your ethereum address here.
Please note, iOS and Android users of the Blockchain.com Wallet will still see 'USD PAX' in the app until the next update.