Find out if/which BLAS library is used by Numpy
numpy.show_config() just tells that info is not available on my Debian Linux. 
However /usr/lib/python3/dist-packages/scipy/lib has a subdirectory for blas which may tell you what you want. There are a couple of test programs for BLAS in subdirectory tests.
Hope this helps.
I use numpy and scipy in different environments (MacOS, Ubuntu, RedHat).
Usually I install numpy by using the package manager that is available (e.g., mac ports, apt, yum).
However, if you don't compile Numpy manually, how can you be sure that it uses a BLAS library? Using mac ports, ATLAS is installed as a dependency. However, I am not sure if it is really used. When I perform a simple benchmark, the numpy.dot() function requires approx. 2 times the time than a dot product that is computed using the Eigen C++ library. I am not sure if this is a reasonable result.