Black Iron Oxide pigment is a non bleeding pigment that is suitable for opaque or white melt and pour soaps and it is also stable in cold process soaps.  If you use it in clear bases it may give a speckled effect.  Depending on how much you add, you can create shades of grey through to black.

It is best to mix this first with either a carrier oil for cold process soaps or with distilled water or glycerine for melt and pour soaps.  If you add it straight into the base, it will probably clump up and look very messy.  Either way it does need to be mixed in well before adding to your base.