Black Peppermint

Was browsing your site, nice range of mints, but I couldn't tell if you had the one I was most seeking. It's usually called black mint, it has dark red stems and darker green leaves than normal peppermint, and isn't as vigorous.
I have fond memories of it from my grandmother's farm in Northern Indiana.

There are a lot of variants in circulation so it is just about impossible to ensure that our peppermint is exactly what you want. It is "darker" in colour than most other mints so it qualifies a "black" peppermint, but it may not be what your grandmother grew.
I should also point out that mints can look different when grown in a greenhouse in a pot as compared to out in the garden. Outdoors then can take on a darker colour just by virtue of the stronger light. So even if you first look at our plants you might think that they are not "black" enough. Not until they are planted out and established might they develop into something similar to your grandmother's mints.

Well, how about this then: which of your varieties are:
A) The most pungent

Peppermint is easily one of the most pungent. "Chocolate" peppermint is very similar (in fact hard to distinguish from the regular variety). Most of the other mints have quite different aromas and flavours compared to the classic peppermint.

B) Best suited to window container growing?

The peppermints can be grown in containers but by virtue of their rangy, fast growing habit they get crowded in a container quickly. When they get crowded and you can't pot up to a larger size you need to break up the root mass and repot portion in fresh soil. Typically this might happen once or twice a season, depending on many factors such as pot size, growing conditions, etc.
There are other mints that have a more restrained growth habit and thus are better behaved in pots. An excellent example is english mint. It has a wonderful spearmint-like flavour and taste, but it is not at all as pungent as peppermint. It is used for different purposes: it is a classic for english pea sauce, for example. It can be used for tea, like peppermint, though the flavour is quite different.

I'm not so concerned at getting exactly what she had as much as I am in getting something that will be at least as dramatically pungent and refreshing, and I want to grow it inside.

To be safe, stick with our peppermint. But you may want to try a few different varieties and select for yourself which is the best for you. In that case, include english mint, chocolate peppermint, japanese mint, scotch spearmint, and swiss mint.
Hope that helps.