C can compile to WASM, but I have no clue about external tools like BLAS. I'd assume that things like Numpy are able to work without tools like BLAS, but you might also be able to compile the essential ones to WASM as well.
NumPy can work without libraries like BLAS or LAPACK, but it will be sloooooooooow. There are C versions of those libraries however, maybe they are using those?