Well, here is my my two penn'eth
I have kept ferrets for 30 years and have had all sorts including pure wild cage caught polecats. I have seen many dark animals but none darker that Romany's above, that seems to be the darkest the putorius strain will go.
I have seen the mink outcross (putorius/vison) but as already stated no line breeding can take place from such a forced cross as the offspring are infertile - althought the scope for breeding almost black or even silver colours is possible.
My own experience, wild pure wild polecats and half crosses are not successful - the result