No, we actually don't need BLAS as a requirement. We fully isolate BLAS / LAPACK behind our own interface, which is disabled during this particular build. However, we do use GSL, which pulls in a version of BLAS. For some reason there seems to be a mismatch between the version of BLAS used at build and runtime when listing GSL as a host and run dependency. (See the MKL issue earlier.) This only appears to happen for certain builds, i.e. OS and Python version.