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Okay so to close the question:

The Problem was that opencv needs both Lapack with the normal blas and 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.