This depends on most networks which utilize a blockchain, but not on all.

There are a lot of projects with either a 'permissioned' blockchain, which does only allow participants based on some properties, decided by a central authority.
Or projects which 'seem' to be decentralized, but in fact are extremely centralized (e.g. NEO).


While a blockchain only is useful and necessary in a decentralized and trustless network, a lot of projects are just using it as a buzz word to attract more user / investors.
It is not the 'blockchain' but the network topology which makes a network/system decentralized.