I'd like to share this comment from Ping, author of Webcomic Finds and creator of comics like The Longest Sojourn :
"Black and White is harder to do than colour by far, IMHO.
"It took me a while, but I'm finally learning that the difference between doing colour and doing black and white (or any monochrome work) is that when the art is meant for colour, you draw LINES. If the art is meant for BnW you draw (ink) SHAPES....
"The bottomline is: doing good black and white requires the artist understand very well the concept of positive and negative space, and how to use it. Sadly most people don't bother studying this facet of art theory anymore.
"It's not just a fancy art term for snotty fine artists; it's an immensely practical skill and there's a very good reason why it is a required study in most formal art curriculum."
Wish I wrote that.
Here's a link where you can bone up on negative and positive space. I'm surprised I couldn't find more, so if you find a good one, please send it here. One discussing the concept in relation to comics would be especially helpful.
Armchair Paint Classes - Quick, but deep, single page lesson