Just off the top of my head...I think you need to create a stub file and then define the categories you want to include (or maybe it's exclude). Then just make sure the categories from your blog b don't get excluded (or maybe it's included) in that list.

So you won't actually use the Blog All as your home page, you'll use the stub file which is pulling in posts (based upon the categories) from everywhere else.

Both including and excluding are possible with excluding being the more future proof. Check this [url=http://forums.b2evolution.net/viewtopic.php?t=11198&start=0&postdays=0&postorder=asc&highlight=]post[/url] for more details.

Good luck