Okay so to close the question:
The Problem was that opencv needs both
Lapack with the normal
cblas if you are using the
ceres-solver. However, the opencv build only links
cublas, which apparently lacks support for some needed functions of cblas.
One solution to this Problem was to manually link
cblas by adding
CMAKE_EXE_LINKER_FLAGS=-lcblas to the
cmake call in the PKGBUILD file.
It is probably possible to circumvent this problem altogether by building all dependencies manually with forced
cublas support. However, that is tedious and not always possible since
cublas is only a partial port.
Thanks again to Philippos, who helped me narrrow the problem down.