An 8mm black pearl first of all is not typically of South Sea origin. 8mm pearls are most common in Akoya, and black would be an enhanced color. Black South Sea pearls are produced in the size, however, but not in abundance. They are commonly referred to as South Sea Tahitian pearls, however. The rarity, although, is not a factor. Black pearls are much more prevalent than white, as there are many more South Sea Tahitian pearl farms than there are 'Pinctada Maxima', or white and Golden South Sea pearl farms.
So IF the pearl is a South Sea Tahitian and not an Akoya, and if the pearl is round, without flaws, it would wholesale for approximately $20 per pearl. This in turn would retail for approximately $70. This is for a top-quality pearl, however.