What features are needed for censorship resistant blockchain? In what situation can you say it's not censorship resistant?
This is a very overarching topic with many answers covering many facets of the Bitcoin / blockchain infrastructure. The network-level is considered by multiple communities to be the enforcer of censorship as it pertains to digital content and transactions.
In Bitcoin we would need to examine the network level and how enforcement of network decisions are made. First code changes are committed and either denied or merged to the actual code base. It is then up to the mining community to upgrade to that newer software version. The network upgrades to new changes when the majority of miners are running that new version.
If the overwhelming majority of mining is consolidated by one actor, or a nation-state, they could effectively censor your transactions by imposing limiting factors on your wallet.
This is only a major issues in Bitcoin and Bitcoin-derived currencies because of the public nature of transactions.