Most people feel that blockchain technology is decentralized technology. Since it is considered as a distributed, and even public or digital ledger, that is commonly used for recording transactions through many systems, there is a doubt that it can be considered as a decentralized technology. Also, the records the presents in blockchain technology can't be altered retroactively without doing alterations to any of the subsequent blocks. 
Blockchain is a database and not really a decentralized technology. However there are mainly two types of blockchain - public and private. Public blockchain can be considered as an example of decentralized technology. Private blockchains are driven by its owners so they are centralized in nature.
The owner can add any rule to the private blockchain. For public blockchain, it needs approval from majority of the users.
Any alterations in blockchain database don't erase the previous records. Rather it creates a chronological order to make the change.
Hope this helps!