Open loan with me. Do not lend unless sufficient collateral is secured.
You mean there is an outstanding loan with you? Is the repayment date over yet?
I can't sign a message because the address is from blockchain.com's wallet, but I can prove that this address had been in use since 2016.
I guess you can if you still have the access of the wallet.
Get the seeds and recover the account using a desktop wallet then sign the address. It should work.