Blender officially supports only the proprietary drivers from both AMD (for GCN cards: AMDGPU; For pre-GCN cards: ATI) and NVIDIA. After installing the proprietary drivers, one can select the graphics card as a compute device under Edit > Preferences… > System .
https://wiki.archlinux.org/title/Blender
Though it looks like it might work with open source drivers if you install opencl-amd from AUR, that’s the package I have to use to get opencl to work with Darktable.