Bitcoin is not Blockchain (and vice versa). Both appear together a lot in talks and various papers but fundamentally they are two separate things:
A blockchain is a distributed database of blocks of transactions that have been verified and added to the chain of blocks
The technology places trust and authority in a decentralised network rather than in a powerful central institution: With blockchain, transactions become decentralised and distributed, cutting out the middleman (Centralised-financial institutions in the case of currency) and becoming self governing through peer-to-peer distributed computers.
Immutable - the blockchain cannot be hacked/changed. When a Miner solves the computational puzzle it publishes the latest block to the blockchain and all the other computers synchronise the blockchain db. Blocks tend to be appended at ten minute intervals. An attack on the blockchain to change a previous transaction would require massive computational energy carried out in a ten minute window and then to synchronise and update all peer-to-peer blockchain dbs - realistically, an impossible feat.