A high pass filter applied in blend mode normal?
No strange it is grey.
You apply high pass filters using soft light or linear light.
Blending modes do not work the same way in darktable as they do in PS.
It is due to darktable working in linear RGB space, were gray values are lower, about 18% (or whatever you adjusted.
In PS you work usually in AdobeRGB or sRGB that are spaces with a gamma curve applied (from 1.8 to 2.2) and so gray is about 50%, the middle of the histogram.
Blending modes (many of them) are formulated with a 50% gray, and pivot about that value.
If you want blending modes to work more like they do in PS, you have to apply that effect after filmic of base curve. Ideally after gamma compensation (after output profile, but I think you cannot make adjustments after that point).
I have to test it more in depth, but I have the impression that they not work the same way even if applied after the base curve/filmic.
We have talked long about it here:
Yes, there are some interface improvement that can be done. Things we are used to in other programs (metadata info overlay you can activate and deactivate when you view an image full screen, more info about important metadata in the miniatures lightable, name of file, more consistent working of sliders in some modules…)
But right now it is quite usable, once you get used to the interface.
darktable is undergoing a huge change in this version, and will have many changes in future versions to adapt to the change from color LAB centric workflow to linear RGB.
We have to keep in mind that this great software is developed by a small group of enthusiast developers in their spare time.
Thanks to all of them.