The reference implementation is written in Fortran, but I'm not sure about the implementations actually used. OpenBLAS has fortran components, but I believe cuBLAS and MKL are vast majority C/C++.

This is a very fair challenge. My understanding was that the main ones in use were fortran and it is very interesting to see that may not be the case. Love to see any benchmark or other comparisons on the options, if you know of any good links.

cuBLAS is not Fortran because it targets NVIDIA GPUs. Despite what the grandparent says, MKL does include some Fortran in its codebase. Most other vendor-provided BLAS and LAPACK libraries are based on the reference Fortran implementation. Even OpenBLAS, the open-source reimplementation, has large parts written in Fortran.