I have a site with several blogs. If I enable the blog search on one of them (settings > General > Show Blog Search Box?) a search using this box lists results from all blogs on the site. I was expecting it to search just this particular one. Replicated on the demo site.
Bug? Feature request? It would seem to be the desired behaviour, as a search across all blogs can easily be achieved with the Site Search component, while the implication of switching this on within one blog is that it will be contained (and that's what we need in this instance).