For some reason, the unicorn's mannerisms remind me of the King of All Cosmos from Katamari Damacy. I have the sound off at work, so I don't know if he's speaking in scratches.
From a purely pragmatic standpoint, this 404 page isn't incredibly useful. It doesn't provide help on finding the page you were looking for, it doesn't link back to the homepage until you click alllll the way through, and it encourages the user to click the unicorn instead of interacting with the site in a more meaningful (and revenue-producing?) way. That said, for some companies style, image, branding, etc are worth a lot, so having this easter egg could be worth the cost. I'm not familiar enough with Blippy to judge.
If they knew what page you were looking for, they would be much better off redirecting you to it. A 404 page is an absolute last resort—a "we have no idea what you want, so here's a unicorn with a double-rainbow on its head" sort of page. I guess, at most, they could include a little form to let you tell them what you expected to see... but I don't imagine too many people filling it out anyway.
