There are three primary differences between these two beautiful black pearl types:
1) Black Akoya pearls are cultured saltwater pearls farmed in Japan and China.
Black Freshwater pearls are grown exclusively in China in man-made lakes and ponds.
2) Black Akoya pearls are perfectly round in shape.
Black Freshwater pearls will be slightly off-round to ovalish in shape.
3) Black Akoya pearls will feature a strong, Midnight Blue-Black hue.
Black Freshwater pearls display much more iridescent, shimmering Peacock colors.
Check out these pictures, which I think really illustrate their differences beautifully:
Black Freshwater Pearls Display a Variety of Iridescent Colors, However Most Remain in the Peacock to Denim-Blue Violet Range.
Black Akoya Pearls are Perfectly Round in Shape, and Typically Display Midnight Blue to Greenish Black Overtones.