Quote from: deepceleron on December 03, 2013, 06:14:00 PM

You can trust what's in a saved-by-bitcoin blockchain and just parse it. The next step in complexity is called "rewrite Bitcoin".

Apparently bitcoind writes blocks to blk*.dat files so long as they demonstrate sufficient proof-of-work. Beyond that, you're left to validating the transactions and discerning the most valid chain yourself.

http://bitcoin.stackexchange.com/questions/17801/does-bitcoind-download-bad-blocks/17804#17804

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Bitcoin will store a block on disk as soon as it has verified its proof-of-work.

Blocks are stored append-only in the block files, and never deleted or overwritten. You cannot judge from the block files which are considered valid, or which are considered the currently best chain - that information is kept in the block index database.