the "key" is a very simple algorithm, that is obviously not illegal

My point is that you could unknowningly store illegal data, without blockchain being involved.

Anyone sharing the data by torrenting an apparently legal file, reposting an image, or quoting a text could redistribute potentially illegal data.

This problem is not constrained to blockchain.

The problem with blockchain is that a full node will be knowingly storing and distributing illegal content (or at least is likely to held to be, once the chain is widely known to be infested with such content.) The only way to.stop is to stop storing and interacting with the chain.

With a torrent, you could be unknowingly doing so, but very easily stop doing so with the particular content once you discovered it (without abandoning torrents altogether), which may or not be a complete bar to liability but would probably result in generally more lenient treatment in any situation other than the government is targeting you and the actual offense is pure pretext, in which case the content hardly matters.

And which subsets of illegal data are legal?