[bluecalm]

Publish a hash somewhere you can prove a date later, like a newspaper or w/e. Then if at any point you publish the text producing this hash you've just proven you had the text at the point of publishing the hash. Blockchain not needed.

[icebraining]

Or a letter to a fellow scientist:

In July of 1610 Galileo was still making discoveries faster than he could publish descriptions of them. On the 25th he discovered that Saturn was apparently situated between two smaller companions that always moved together. Wanting to establish his priority of discovery, but not yet ready to reveal what he had found, he sent to Kepler (and others) the following jumble of letters, which he informed them was a coded description of his latest discovery (...)

http://www.mathpages.com/home/kmath151/kmath151.htm