Backing up your Blockchain.info wallet is critical and can be done by following this guide. Creating the backup phrase will secure your bitcoin and ethereum balances.
You also have the option of exporting the individual private key for your ethereum address. This is useful if you accidentally sent ethereum based tokens (known as ERC20 tokens) to your Blockchain wallet. At this time we do not support sending tokens or showing token balances in the Blockchain wallet, which you can read about in greater detail here.
To export your ethereum private key start by clicking ETHER in the lefthand menu:
You will see the main transaction page for the ethereum section of your wallet. To the top right you can see Export Private Key:
Press Export Private Key and acknowledge the following message:
Press Continue and the private key will be displayed. As stated in the warning you should never share this information with anyone. Access to a private key means access to the funds associated with the public address.