Hi :)
I just noticed that the Bump- Command (and maybe other "Layers") affect the "other" Material in a Blendmaterial whereas the Diffuse- Slot is "splitted" fine. To show you an example I put a Checker- Map on a Wall witz Dirt Details. The Dirt is seperated, the Bump (little "waves") can be seen on the concrete Wall.
Greedz :)