You need to use wallet software that will show you unconfirmed transactions and allow you to select those transactions’ outputs for spending.

My favorite such wallet is Sparrow. Electrum can also do this. Not sure about other wallets.

If it was me, I would:

1. Install Sparrow on a trustworthy desktop/laptop*

2. Create a new wallet in Sparrow using whatever signers (hardware wallets) you use for cold storage.

3. Confirm that you can see the expected cold storage info (address matches the one you sent to from Blue Wallet, funds are the expected amount, etc.)

4. Use Sparrow to create the CPFP transaction.

If you’ve never used Sparrow, probably best to practice by making a dummy wallet and playing around with small amounts first. Creating a CPFP transaction is a somewhat advanced maneuver.

* Verifying GPG signature as is customary.