In case of the Bitcoin split - when holding BTC on blockchain.info - will I receive coins on both forks?
Yes, blockchain generates address deterministically as per BIP 44, and uses mnemonic backup phrases as per BIP 39, so you can recover all your keys using that mnemonic. It is compatible with any other wallet which follows BIP 39 and 44, and because it allows you to regenerate your private keys, it is equivalent to backing up the keys themselves. So you should be safe.

References:


https://support.blockchain.com/hc/en-us/articles/115001298143-Your-Recovery-Phrase-The-Failsafe