Normal Map Z axis (Blue Channel) control doesn't work ?

Using my decal normal map


Now without my normal map decal ( it uses the surface normal, right ?)

But the big question is, how can I blend both and control the intensity to achieve that “absorption” effect without having to touch every surface material ?

Like this example (I made a custom surface setup, increasing its normal intensity based on buffer)
See how rough in the corner it looks ?