Fortunately I found my wallet Id for blockchain.com wallet and I can now access that wallet but yet I can't find a way to sign a message from blockchain.com.
Open the "Wallets & Addresses" menu on settings, and from the address you want to sign the addres, More Options--->Sign
You could also export the private key (same tab, but private key instead of sign) and sign it on electrum (for example).
Hope that helps you