First of all, coinbase does not have a features of importing of private keys, you can only use the address that they had provided to you when you first created your account. Why not just make a transaction that will send those Bitcoin from your blockchain.info wallet to your coinbase Bitcoin wallet? And for your information, even though coinbase is one of the most trusted Bitcoin exchange, it is still not recommended to store your Bitcoins or any cryptocurrency on exchanges especially if you will hold huge amount of it. Why not just create a paper wallet or buy a hardware wallet for better security?