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You don't need to log in at all if you have the .json file.
Go to this link:
https://login.blockchain.com/wallet/import-walletDrag and drop your .json file in to the box
Enter your first password when prompted
Enter your second password when prompted
You will then be asked to set a new password for the wallet
You will then be redirected to a new log in page for the new wallet - enter the password you just set
You can then use the wallet to transfer the funds or export the seed phrase or individual private keys in to Electrum. Either way, you should empty out the wallet as soon as you have restored it and send the coins to a new, secured wallet.