I'd assume both blogs were SEO'd in the same way. It's pretty clear that the methods you used are no longer liked by Google. Either inbound link issues/devaluation or over optimized on-site factors. But over optimization will only drop you a few pages usually, not 10.
But if it's only a day, wait a while. Could come back.
BTW, Matt Cutts reads this forum and takes notes of every domain then feeds that to the Google Super Computer at night.
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