I have two keys to import, one starts with 'D' the other with 'C', same error with both.
Your private keys aren't in Wallet Import Format, they are base58 which doesn't work with Bitcoin Core.
On blockchain.info, go to import/export > export unencrypted and on the dropdown "select private key format" select "Bitcoin-Qt Format". They should begin with 5, K or L.
for your help.