Sorry to resurrect an old thread but this is the newest about this sort of thing. After much lurking, I've gathered that there's a disdain for dark trousers in general. Along this sentiment, people have mentioned that because we generally want contrast between trousers and blazer/sportscoats, darker trousers mean lighter sportscoats, and several have said they dislike that combination.
Obviously we generally prefer to draw eyes more towards our chest/head area, so many argue that a color like the much hated black draw all the attention/suck all the color/etc. Black is a stark color, it is not warm. Furthermore it stands in contrast to most peoples features, whereas the lighter tan or mid gray is more neutral.
Then here is my theory: none of the people who hate dark trousers have dark/black hair.
That is to say, for someone with dark features/hair, a very dark color actually is more neutral and it is the light colors that stand out, and therefore it's relatively more appropriate for them to use darker colors where they want muted pieces, and use lighter colors when they want pieces to stand out. In this case, dark trousers and lighter colored blazer.
I've always thought some of the "rules" here are exclusionary; black suits are generally more accepted in Asian countries. I'm sure lots of SFers would just reply saying that's because they dress worse. Maybe that's partially true but maybe there's also more?