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To me it seems like these keys were shared on a site somewhere and when people Googled for free blurity key or whatever these came up.

If that were true the author would have been able to find the keys, but he said Google failed him.

It's possible the serial was posted on a dark net, such as Tor.

Maybe they didn't search on Google then, but another forum or site that isn't indexed by Google.

Maybe the invalid key was posted on a warez site and everybody was trying to use it.

Besides a keygen, software might be bundled with a list of keys to use when downloading from certain sites.