The BLIS-like Library Instantiation Software (BLIS) is the current gold standard for open source BLAS libraries. https://github.com/flame/blis It is not as fast as MKL (although close) but faster than OpenBLAS, a fork of the legendary GotoBLAS, on essentially all CPUs (and much faster on the latest architectures including Intel, AMD, and ARM). It is well maintained.
ACML, mentioned in another answer, no longer exists. AMD now uses open source software as part of their ACL (AMD Compute Library) software stack. BLIS is what is part of that software stack: https://developer.amd.com/amd-cpu-libraries/blas-library/.
Caviat: poster is part of the BLIS project. The above claims are well-documented.
(Comment added later: Didn't notice the ".NET". Unfortunately BLIS is not yet supported well for Windows.)
