There is a typo #elif BLAS_BLAS_IS_64BIT in line 61 in blaze/math/blas/Types.h that causes blas_int_t to remain 32 bit if #define BLAZE_BLAS_IS_64BIT 1 is set.
Thanks a lot for reporting this bug. This is a very good and valuable catch indeed, since we didn’t realize that despite explicitly testing it against several BLAS libraries. We’ll fix the issue as quickly as possible. Thanks again,