first pass at automatic aggregation for the fromPages option
If blog A also shows posts from blog B, and blog B also shows posts from blog C, then blog A gets all of the above. This is intuitive because the author of blog A shouldn't have to care what blog B is composed of. Also, tag filtering works cumulatively. So if blog B shows only "dogs" posts from blog C, and blog A shows only "cats" posts from blog B, then only blog C posts individually tagged with *both* "cats" and "dogs" show up in blog A.

This also works correctly with blog widgets.