Material morphing

Material morphing for say bumpmaps on a character...if i use the material morph and link it the a morpher modifier when i say blend a frown bumpmap and a smile mouth bump together i can't get them to go fully black [100%] on their own they go black but when you combine material morphs you can't get full black....
So what's the workflow for doing this for character animatios where you want to link geometric morphs and material morphs together? combining 2 black and white bump maps with the morpher give a grey output even at 100%

The main thing here is how to mix to morphs maps..say the bumps on a face and have them mix over to fully bumped...without having to create a specific pre mixed extra map from say photoshop but to use the 2 maps to mix over...I hope you understand with what I mean.