Your Passive Rewards Account is custodial (unlike the funds you store in your DeFi Wallet, which is non-custodial), so it is not backed up by your wallet backup phrase.
Your Passive Rewards Account is associated with your identity profile. If you lose access to your Blockchain.com, all you need to do is use your backup phrase to recover your wallet and get re-verified in your new wallet to recover your Rewards Account funds.
Please note: the Passive Rewards Account is not a bank deposit and is not backed by any governmental deposit protection regime. Your rights in connection with the Rewards Account are governed by the contractual relationship you enter into with Blockchain.com via the User Agreement.