The purpose of the blockchain is to be a shared append-only data structure. There are a number of nodes who have copies of what is supposed to be exactly the same data (but see below). The nodes are all constantly telling each other what they believe to be the most up-to-date version of the data. The purpose of the blockchain is to support a consensus-finding algorithm by which the nodes may start out disagreeing on the contents of the most recent few blocks in the log, but eventually converge to a consensus, at least for relatively old blocks.
This convergence occurs even if some of the nodes are evil or faulty or out-of-touch, provided that these bad nodes control less than 50% of the total computing power of all of the nodes.
What would you say that is? Real question.