When setting up PostProcessSettings Blend Weight doesn't affect on PostProcess material appearance by default. There is two solutions.
The first is simply to create your own Blendable in c++ or Blueprint and implement desired functionality as its suggested by Epic (https://docs.unrealengine.com/latest/INT/Engine/Rendering/PostProcessEffects/Blendables/index.html)
The second is to set-up right settings in master material and material instances of PPM. I've found that Blend Weight default behaviour is performing interpolation between master and instance material parameter values. It's a key to this solution. When Developing a shader, it is necessary to pick out all parameters that changing shader appearance and set its values in master material so master material doesnt
