The vast majority of BLAS and LAPACK are written in Fortran (at least, the ones provided by netlib.org). Github also reports that OpenBLAS is around 50% Fortran, so there's quite a bit there too [1]. Usually when you call BLAS functions from C you're really calling a cBLAS wrapper to some Fortran function. This is generally why you see -lgfortran -lf77blas when you compile programs that require numerical tools.

[1] https://github.com/xianyi/OpenBLAS

At least in OpenBLAS, most (all?) of the BLAS routines are C functions that call kernels written in assembly, but many of the LAPACK routines are the Netlib reference Fortran versions.