Hiding show buildings by painting them blue is half a solution. For the vast amount of what you see -- it works pretty well (as long as you're not seeing the side with downspouts). Yet the problem is that our eyes are naturally always looking for edges to discern shapes and identify things. The edge where the fake blue and real blue sky meet grabs our attention and the illusion collapses. And then it kind of feels just a bit insulting, like "do they think we're
that dumb?" The effort they put into the illusion is a signal of just how intelligent they feel we are.
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A more advanced version of this illusion obfuscates that edge with an intermediating distraction. Logs for Berk (because there are space-frame log structures throughout that land) or clouds for Monstropolis, for example.
Having rounded edges also would help for the same reason the inside curves of a photography studio -- a cyclorama -- are rounded, so as not to be perceived. It hides the transitions between planes.
The other alternative is a continuous scenic backdrop with a thematic shaped profile extending above the building. For example, the mountains of Disneyland's Toontown. For Monsters, Inc. they can do the backdrop like that with many smokestacks and water towers and have a bumpy smoke edge at the top extending above the otherwise flat roof of the show building.
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In an even more advanced (though experimental, I haven't yet seen this) version, instead of blue paint they would use mirrored surface, perhaps angled slightly up to dynamically reflect the actual color of the sky, combined with edge-concealing, to make the building "disappear":
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There are opportunities to do better than just blue paint, and it will be interesting to see what they do at Monstropolis.