It is a Merkle DAG of transactions secured by cryptographic protocols. It very much is a blockchain -- it just doesn't use proof of work as a mechanism to exclude bad actors. The fact that it uses UNLs and consensus to achieve a consistent view of valid transactions does not make Ripple "not a blockchain" and, as XRP relies on cryptographic mechanisms to secure rights of ownership on that blockchain, XRP, the native Ripple asset, is by definition a cryptocurrency in the colloquial sense.
