As far as I know, Black aros are born with a "Pinguin suit" and then as soon as they begin to grow the color begins to wash out toward the fins. You can expect color changes to start by the time the fish is 3 inches.
One thing to be aware of, If your aro has a yolk sack when you buy it, the father was most likely killed and the babies were taken from him. 1, you shouldn't support fish taken in this manner, 2 they are harder to raise then aros that are allowed to grow naturally.