That's a telltale that the bump value is too strong, and that it points to the displacement realm rather. Basically, if you have black zones away from the terminator (edge of shadow) when using bump, then you are doing it wrong. I understand that the here material is reflective though.
I know that well for I have explored procedural anisotropy, à la Passionated
. The narrower the map details (ie the higher the frequency), the lower the gain. Twice the frequency, half the value. That simple. It's illustrated in the shader, where gain is divided by frequency.
By the way, displacement is just perfect now, performance-wise. Done right !
PS: example: if the distance between two peaks is 2mm, then a gain of 1mm will average their slope at ~45°.