Few introductions to blockchains manage to clearly state what blockchains do. This is odd, because they do only one thing.
Blockchains allow a group of entities (individuals or organizations) to accumulate an ordered list of shared records over time, whose tamper resistance they can trust - even if they do not trust each other.
That is it. That is the only new thing which blockchains accomplish. It turns out however that given the right context, this one little thing happens to be a big deal.