BLAS generally means the routines written in Fortran. CBLAS is simply a wrapper around these routines, providing a C interface to call them. The CBLAS performance should be virtually identical to the Fortran BLAS performance. In most cases, MAGMA actually directly calls the Fortran 77 BLAS interface, but for a few routines (such as zdot), it is not well defined how to call the Fortran interface. Because Fortran doesn't mandate how function values are returned, so there are several different conventions. For these, we use CBLAS as it is more portable. These are not performance-critical routines.