Fishie, if you want a general gauge of how large the black will be, look at the silver aro in the HFF Albany tank. If I'm not mistaken, blacks grow to about the same size as silvers but with different coloration.
On another note, there's nothing to feel 'uneasy' about with having a solo aro in the tank. Personally, I feel that having one aro per tank not only gives the fish all the space it wants, but it is also easier to enjoy viewing the fish. Having a huge tank full of aros looks cool, but you don't really get to examine each one and build a bond with them. Aros are actually very personable fish, as I've recently found out. Mine is a shy eater lol, if there is anybody else in the room with me during feeding time, he refuses to eat. But when I follow my usual routine and feed him without anyone in the room, he chows down like crazy. Spend time in front of the tank so that the fish will recognise you and identify you with feeding, then he will follow you in front of the tank.