How may I force a blog post to automatically post onto another blog only if a certain tag is used?

I believe you may be looking for this:  Sharing Content Across Groups  

Hi, TTom Paolucci . This was very helpful. One question. If we want to pull complete blogs from two sources, but only use tags from another source, is it possible to configure that widget to accomplish both of these goals without restricting the blogs that don't require tags?

I see, you are referring to Mirroring Content from an External Blog and I don't know of a way to filter those posts.