Okay, with the caveat that I'm not a professional stylist, I am an attorney who has spent a lot of time with male attorneys working at the top law firms in the world, and I look at their suits quite a bit--here's my take.
I would much rather see someone in a higher-quality black suit than in a lower-quality suit of any other color. I think that quality and fit are really important when it comes to a men's suit. It sounds like this is your significant other's first and only suit. As such, I actually don't think there is anything wrong with it being black. He'll need to wear it not only for interviews, but also for occasions like weddings and funerals, right? Black is always appropriate for those. If this is going to be his only suit, I would absolutely prefer black over a light color (like a light grey or a tan), because lighter colored suits often look inappropriate at events like funerals, and can sometimes look seasonal.
My boyfriend is also an attorney at one of those big-time law firms, and he has several suits: charcoal grey, navy blue, navy blue pinstripe and black. He wears the black suit for work, interviews, etc., in the same way that he wears the rest, and I know of many other attorneys who do the same. (Also, as a female attorney, I own more than one black suit, and pretty much every other female attorney I know does as well.)
As for the stores you have listed, I would definitely go with a department store over BR or Express. My experience in looking at suits from BR and Express is that the fabric doesn't feel as nice, the suits don't seem to fall as well, and just don't look as good. You mentioned Nordstrom Rack--if you have outlets near you, you should also try going to Off 5th (the Saks outlet) and Neiman Marcus Last Call, if those stores are available in your area. Filene's Basement could also be a good try, although some of the suits there will be of higher quality than others. You should be able to find great deals at all of those stores--in fact, last week Off 5th was having a buy one get one free sale on men's suits!
