If you did not have any bitcoin/BCH in your wallet at the time of fork (August 1, block 478558) then you will be unable to use BCH within the Blockchain wallet. The Blockchain wallet is not meant to be used as a BCH wallet for new or existing users who don't have any BCH. We are only making these funds available within our wallet to users who had bitcoin/BCH at the time of the fork. There is no option to import BCH addresses into your Blockchain wallet.
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