[carver]

It depends on the context.

Ethereum is a platform that uses a blockchain to build and record consensus. Conceptually, the blockchain is a technology underpinning each cryptocurrency. Across the many cryptocurrencies in use, there are many blockchains keeping track of who owns what.

It is never correct to refer to a capitalized "Blockchain," or refer to it singularly without articles (a or the), imho. We don't refer to other data structures that way, like a priority queue.