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.
I have no idea why blockchain.info uses base58 as the default format, they really should change that to WIF. Almost no clients use base58.