There is no rule. I think it is for StackExchange to clarify, if they choose to. I don't care, one way or the other. Rebecca Chernoff, et al., seem to care, though, and would prefer it didn't happen -- "content farming" was a phrase used. So as far as I'm concerned, if the content is high quality, and original to the CSTheory blog, it is fine with me if it is also "simultaneously original" somewhere else.
I would not want to repost something already posted somewhere else a year ago. But material that appears the same day on the community blog and someone's personal blog seems fine to me. I don't think it matters where it appears first.
One possible exception: the community blog is on the TOC Blog Aggregator. If the writer's personal blog is, also, that would cause two copies of the same post to show up in everyone's feed, which seems a bit rude. So I'd want to discuss it with Joe if we are ever in that situation.