blockchain is neither centralized nor decentralized. it is just a database.
the blockchain technology refers to technologies that use blockchain as their database to store "data", bitcoin uses it in a decentralized way but there are also altcons that use the same technology but in a centralized way.
Also, the records the presents in blockchain technology can't be altered retroactively without doing alterations to any of the subsequent blocks. ?
that is the definition of a "chain", you can't change one item (eg. a block) in that chain without breaking the whole chain from that point forward.