The term Blockchain has become a fluid concept, changing with the industry. At Metal we refer to the Ethereum blockchain primarily, as our token is on the Ethereum Blockchain. Since it's a complex subject to explain in a FAQ article we'd like to send you to a few resources for more information.
Ethereum Blockchain and History: https://en.wikipedia.org/wiki/Ethereum
Bitcoin Blockchain and History: https://en.wikipedia.org/wiki/Bitcoin
General Blockchain Information and History: https://en.wikipedia.org/wiki/Blockchain