How can blockchain.info show the output address of a transaction when the output address has never been used in any other transaction?
I assume that you're talking about a Pay-to-pubkeyhash (P2PKH) transaction, which is the most common transaction type on the network at this time.

The pubkey script of a P2PKH transaction output contains the hash of the public key (what you call HASH160):
OP_DUP OP_HASH160 &lt;PubkeyHash&gt; OP_EQUALVERIFY OP_CHECKSIG

Said PubkeyHash can be converted into the address by prepending two zeroes and expressing it in base58check, a more human readable format.