I am sorry,
I still have no idea what you mean.
If the mask is just pure black and pure white (no grey!) and you plug it into the Fac of a Mix Shader, than you get the first Shader for the black parts and the second Shader for the white parts. Nothing should darken or get dull because of the mask.
I’d test the mask on two basic (for instance Emission) Shaders.
You can also insert a Math > Less Than Node just after the mask, to make sure it’s only black and white.
