I'm wondering if this is related to the issue that b.info has where it gives out compressed WIF keys for uncompressed addresses (or vice versa) when you export private keys. Is it possible that their system has sent the change to an uncompressed change address, but the wallet is using compressed addresses/private keys?

Given that you have an xpub and xprv, you appear to have an HD wallet with blockchain.info... so I'm guessing you should also have the 12 word recovery backup as well, correct?

If so, you could try using the BIP39 Mnemonic Code Converter (https://iancoleman.github.io/bip39/ - create offline copy etc), put in your seed, click the BIP44 tab... and see if it creates your addresses... if you see your "receive" address(es), try changing the "External/Internal" value to 1 (BIP44 derivation path: m/44'/0'/0'/1)... that should then generate your "change" addresses...