As it is already said in previous posts, first you should extract your seed phrase from blockchain wallet and import it into Electrum. Then you can use Electrum to extract your private keys.
In order to extract your seed phrase, you should click on "Security Center" on top of the page and then click on "Backup" button.

Note that private keys must always be kept private. You must be the only person in the word who know that private key.
Blockchain wallet is not open source and we don't know whether you are the only person who access those private keys or not. They might have access to your private keys.

I recommend you to no longer use Blockchain wallet.
Instead of importing your blockchain wallet to Electrum, create a new wallet in Electrum and then send all yout bitcoins to the new address Electrum gives you.

Save your private keys in safe place. In this way, you can be sure that you are the only person who access those private keys.

Note:
Download Electrum only from https://electrum.org/ and don't trust any other file.