True, PoS was described first in the Peercoin whitepaper. However Peercoin still needs PoW together with PoS. Blackcoin was the first coin, that went full PoS for validating transactions and maintaining the blockchain. It was said, that the motivation to create Blackcoin was to show, that a stable and secure blockchain can work entirely on PoS alone. For this Blackcoin did a few improvements to the protocol and from there, it caused a list of forks like Stratis.