Use Higgins Black Magic as your black. Truest, deepest waterfast black there is. Spend the money on good brushes. I like simmons blend brushes...squirellhair, synthetic blend. Don't worry about trying to be perfect. You will do hundreds of watercolors before you get one good one.
The most important things for me are this: Paint loose. paint fast, line with a brush, don't worry about actual color until you can blend black super smooth(from true black to paper with a perfect gradient).
If you have more specific questions, please post 'em.