This is commonly known as the projected potential ability; train him correctly and play him often and he'll reach the 'black stars'. If you don't use him enough and/or don't loan him out to play, he won't reach maximum potential. Also you'll notice the black stars will often fluctuate, dependant on how you use him.
The black stars are the opinion of your staff on a player, so you will have a coach who thinks a player has 5 star potential, however they cant be 100% sure. Its like anything in life you can think a player will be excellent but depending on their attitude, training, match time etc, they might not reach it. As a manager its down to you to help the players reach their potential.