The black Elegant Writer pen by Speedball can be bought individually or as a set of 4 from SAA
It is a calligraphy pen with a chisel shaped tip, it can also be used as a drawing pen. Its best used on a cartridge or HP watercolour paper as its a bit scratchy though still doable on NOT watercolour paper.
The ink is watersoluble so if you touch the line with a wet brush the black ink diffuses into the water making washes of aqua grey and sometimes pink. Paint just one side of the line to get the wash to flow in the direction you want. Its a bit unpredictable but fun to do. Its also very portable. All you need to sketch on location is an Elegant Writer pen, a loaded water brush and a sketchbook.
If I don't like the effect I sometimes lighten the tone by painting over with some white acrylic ink.
You can find an example of a water brush here
It's possible to do similar with Quink permanent ink and a dip pen. The colours are generated by water and are a bluish grey and ochre.
Below the pen is used dry for calligraphy.