From Arch:
The openblas package prior to version 0.3.23-2 doesn't ship optimized LAPACK routine and CBLAS/LAPACKE interfaces for compatibility. This decision has been reverted now, and the ability to choose a different default system BLAS/LAPACK implementation while keeping openblas installed is now provided to allow future co-installation of BLIS, ATLAS, etc.
The default BLAS implementation will be used for most packages like NumPy or R. Please install "blas-openblas" and "blas64-openblas" to make OpenBLAS the default BLAS implementation, just like the old behavior.
Unfortunately you will get errors on updating if you currently have OpenBLAS installed as the default BLAS implementation:
error: failed to prepare transaction (could not satisfy dependencies) :: installing openblas (0.3.23-2) breaks dependency 'blas' required by cblas :: installing openblas (0.3.23-2) breaks dependency 'blas' required by lapack
Please append your preferred default BLAS implementation to the regular -Syu command line to get around it. For example:
# pacman -Syu blas-openblas
or
# pacman -Syu blas