I think the confusion stems from permissioned vs permissionless blockchain, and the comment you replied assumed you were talking about one instead of the other. I find the plain term "blockchain" rather vague in that regard.
For precision, it’s good to distinguish between decentralization and permissionlessness as separate axes. Technically speaking the two are commonly positively correlated, but you can build centralized, permissionless systems and decentralized, permissioned systems.
In addition to this github gist, I like http://doyouneedablockchain.com/ .