As I understand, steemit has the "delayed" blockchain. The delay is for 1 month, so you can remove or edit your post. after your post is added to the blockchain, you cannot change it. Every post, every comment forms a block that is recorded forever (until the last computer, supporting it is online).
But steemit doesn't have "classic" blockchain.
Classic blockchain, like Bitcoin blockchain, records every transaction of bitcoin from one user to another between their wallets. After transaction is confirmed by few miners, it is impossible to change it. Actually, once you've sent your bitcoins, you cannot cancel it and bitcoins will move from one wallet to another anyway. It's like huge accountant's grossbook, where everything is recorded. But you can cheat on the grossbook, not with the blockchain.
Every block is created every 10 minutes and cannot be changed.