Long time since the last post - it requires a certain amount of self-discipline to keep blog posts going, especially when so few comment on them. Yes, that's a hint.
Anyway - there's an interesting discussion on the Painters Online forum pages, (www.painters-online.co.uk) started by Alan Owen, a watercolourist friend of all of us. Alan points out that using black - especially, though not exclusively, in watercolour - has been discouraged and generally frowned upon. There are good reasons for this - using black as a routine means of suggesting shadows, for instance, can quickly turn a picture into mud. Black can be mixed - eg from Winsor/Pthalo green and Permanent Rose, or from Ultramarine Blue and Burnt Sienna or Burnt Umber, and in any number of other ways: black is basically the result of a mix between red, yellow and blue; you've got the yellow in Pthalo Green, red and yellow in Burnt Sienna and Burnt Umber. Blacks mixed this way will have far more life, and run much less risk of muddying your colours.
However, adding a small amount of black to other colours can produce very interesting results - take a look at the POL forum for some examples of Alan's colour mixes, especially the results he gets with a little Old Holland Intense Black mixed into Cobalt Blue.
The painting below (I hope it appears below anyway: I've been caught out with files appearing in the wrong place before!) contains a very small amount of black in the blue of the sky; this one was painted with Chromacolour (www.chromacolour.co.uk), a form of acrylic. Without the little bit of black, I couldn't have achieved the colour I wanted from those available to me at the time. So black has its place on the painter's palette - you don't HAVE to use it: but it's not the crime it's sometimes held to be.