in your blockchain.info account, do you have more than one "account"? i am not sure what they call it but it is basically when you create a new group of addresses. they may call it account or wallet.
if that is so, go to
https://iancoleman.io/bip39/ (make a
copy of source and run it offline for security), enter your master private key (string starting with "xprv") in "BIP32 Root Key" text box, then scroll down to "Derivation Path", select BIP32 and in "BIP32 Derivation Path" enter this: m/44'/0'/1' i believe that is how they create a new account but haven't checked their code.
(m/44'/0'/0' is the root path that is used for deriving your keys: m/44'/0'/0'/0 then m/44'/0'/0'/1 and so on. m/44'/0'/1' is another account from the same master key with keys that are coming from paths: m/44'/0'/1'/0 then m/44'/0'/1'/1 and so on.)
scroll down and if you see your addresses here then scroll up and copy the "BIP32 Extended Private Key" text box content (string starting with "xpr") and enter that in your Electrum.
(depending on how many of these "accounts" you have created in your blockchain.info wallet you may need to change the 1 into another number like if you have 3 and want your third account you put m/44'/0'/2' there since indexes are 0 based).
