BlackCoin is a cryptocurrency that uses a proof-of-stake consensus algorithm for securing its network. It was created in 2014 and is designed to be a fast, secure and energy-efficient alternative to traditional proof-of-work cryptocurrencies like Bitcoin. BlackCoin's transactions are verified by stakeholders who are chosen to validate transactions and add them to the blockchain based on the amount of BlackCoin they hold and their ability to prove ownership. This helps to reduce the energy consumption associated with proof-of-work mining and also makes the network more secure against malicious actors. BlackCoin was not the first proof-of-stake blockchain, but it was one of the early adopters of the proof-of-stake consensus algorithm. The concept of proof-of-stake was first proposed in a research paper in 2011, and several other cryptocurrencies, such as Peercoin and NXT, had already implemented the algorithm by the time BlackCoin was created in 2014. However, BlackCoin's implementation of proof-of-stake was notable for its use of a "pure proof-of-stake" mechanism, in which the entire network is secured by stakeholders rather than a combination of stakeholders and miners as was the case with some earlier proof-of-stake cryptocurrencies.
BlackCoin is often considered one of the earliest cryptocurrencies to rely fully on proof-of-stake (PoS) as its consensus algorithm. Proof-of-stake is a different approach to achieving consensus in a blockchain network compared to proof-of-work, which is used by Bitcoin and many other cryptocurrencies. In a proof-of-stake system, validators are chosen to validate transactions and add them to the blockchain based on the amount of the cryptocurrency they hold and their ability to prove ownership. While BlackCoin is one of the earliest cryptocurrencies to rely solely on proof-of-stake, there were earlier cryptocurrencies that used a hybrid approach, combining both proof-of-stake and proof-of-work. For example, Peercoin, which was created in 2012, was one of the first cryptocurrencies to implement proof-of-stake in combination with proof-of-work. It's worth noting that the concept of proof-of-stake was first proposed in a paper by Sunny King and Scott Nadal in 2012, and since then, many other cryptocurrencies have adopted proof-of-stake as their consensus algorithm. However, without more specific data, it is not possible to definitively say which cryptocurrency was the first to rely fully on proof-of-stake.