Can blogs and wikis be searched?

I went to the Wikispaces homepage, and started to experiment with creating a new wiki from scratch. A suggestion came up to go experiment in the "sandbox" wiki first, so I did that. Somewhere in the Sandbox, I read a post where someone said that blogs can be searched by search engines, but wikis can't. This doesn't make sense to me, since entries on Wikipedia come up in Google searches. Does anyone reading this post know for sure whether blogs and / or wikis can be searched by search engines, and if so, whether that can be blocked (other than by paying for a private wiki, that is)?