You can export the private keys of individual addresses, one at a time. Go to settings -> addresses, and for each address you want to export, go to more options -> private key and acknowledge the warning.
There is
no export feature for private keys of blockchain.info since they update there wallet to HD wallets, and you cant even export private keys of individual addresses of it, I don't think if where do you see this tutorial since then. All you can do is import a private keys to blockchain.info. So to OP if you just want to withdraw your coins there then just send it to other address.
Problem is I want to withdraw clams and blockchain.info doesn't even show your clams balance :/
And to get the clams out, I would have to import my wallet.dat into the clamcoin wallet