If blacking out your website, return status code 503 to avoid an SEO hit
searchengineland.com
searchengineland.com
It just so happens that 503 is the semantically correct response for this situation, while other 50x responses are not.
Even better advice would be to serve different content for regular users. This is easily done through User Agent sniffing. You can do it at the app level or web server such as Apache's Rewrite rules.
Is this cloaking? It might be. However, for only a day, there shouldn't be any harm in it.
'Saying "If blacking out your website, return status code 503 to avoid an SEO hit" == "Bring the caviar when you come to the protest camp"'
If you're not willing to demonstrate damage to your situation in your protest, then it doesn't really count as a protest much, does it?