As it name depict, Blockchain is basically a chain of blocks connected to each other forming hashes in the process. Hashing can be defined as a mathematical operation that takes input of large or small data and transform it into a small one of fixed size. it does so by producing alpha numeric digit in a string of text. Input and output of a hash function is unpredictable and dependable on the block content and varies differently from a data to another. Every block hash is unique identifying that particular block. But blocks are chained such that, the hash of the new block depends on the previous block creating an unbreakable chain back to the genesis block.
With the continues effort of referencing old block before new ones are formed, it provides a linkage from genesis to future blocks. The blockchain makes it unable to alter anything without anyone noticing. So, in the case where one wants temper with a block it automatically invalidates the following blocks which means redoing or altering all other blocks of over a million and achieving consensus from majority of all existing nodes of that network which is fairly impossible.
When an altering attempt is made, in security terms it reveals a temper evidence on the blockchain but it is made temper proof through the Proof of work (POW) mechanism. Though you can have a temper-proof blockchain by verifying against an “official” reference but blockchain is made reliably trustless with a POW ( i.e not requiring a central authority to provide an official version ).
Consensus (or 51%) attack is theoretically possible with a high investment in hashing equipment, but this is expensive and is unlikely to be done because it is not a profitable attack on typically sized transactions. Very large transactions face some risk, but requiring multiple blocks to go by to “confirm” a transaction raises the cost of attack to be higher than the the size of the transaction, effectively securing the network. For all practical purposes, older blocks (beyond the few dozen most recent blocks) become permanently immutable.
Hence the claim remains that, blockchain is immutable to some degree and proven a secured network in the recent past years.