I read this thread earlier and have been keeping an eye out around the office since then for people wearing black with colour. It's a big office so lots of outfits to observe.
It seems that it is really quite easy for black trousers with a coloured top to look dowdy. Especially if it's a coloured, patterned blouse or if it involves a waterfall cardigan. And especially if it's paired with sensible flat shoes (sorry).
Black trousers with a grey or beige/camel or muted plain colour top (like khaki or burgundy) looks much better.
Flat shoes look better with cropped (like 7/8 length) trousers.
Silky tops are less likely to look dowdy than a shirt/blouse.
On the other hand, coloured trousers (even if bright or pastel) with a black top is much less likely to look dowdy.
You can definitely, definitely tell expensive and well cut black trousers in good fabrics from cheap polyester ones. The former are easier to wear and look stylish.
All of this is just my opinion.