Black diamonds are just stones with such a high content of inclusions,it makes them appear black,if you looked at one under a microscope with the right lighting,you can indeed see 'clear' patches in them.They are'nt paticularly rare,they are just rarely used in jewellery compared to their whiter cousins,and the fact they are a little more prone to damage.
A black diamond that size is best worn in a pendant,it'll get less 'wear' than a ring stone,and be safer,black diamonds by their nature and the inclusions they possess can be a little fragile,as the strong structure of the diamonds bonding is 'interupted' by the prescence of so many inclusions.