1. Use your 12 word backup phrase from blockchain.com and create a "BIP39" wallet within Electrum. Electrum would then have the same addresses as your blockchain.com wallet and would let you sign using your "blockchain.com" addresses
or
2. Use your 12 word backup phrase from blockchain.com with an (offline) copy of Ian Coleman's Mnemonic code converter. That will give you the private keys for your blockchain.com addresses, you can then use those private keys and import them back into blockchain.com... it should let you sign a message with one of those imported addresses.
NEITHER of these options should be used lightly and without taking adequate precautions to secure your 12 word backup phrase and/or private keys.