I don't recommend the use of any Black or Paynes Grey out of the tube either for oils, acrylic as well as watercolours. The reason?
1. Beginners over use it and actually "kill" the nice vibrant transparent colours. I don't even use it. The preferred method is to mix what you need and to match the mood of the painting, eg, a warm black or a cool black. I prefer to call them very dark colours as very seldom do you get a pure black in nature. Don't be mislead by the "blacks" in a photograph.
2. It is OK to use sepia but also be very careful with these colours for the same reason as above.
3.Chinese what should also NEVER be mixed into the watercolours to lighten them - all you will get is a very chalky and dull look. Being extremely opaque it takes away the transparency of the colours because transparency is what watercolours is all about.