Black obliviously lives a life of a grown up even more than White does XD he has a successful job, he takes care of Ty’s kids, and helps Waseem with his school work so it probably didn’t register to him that smoking is a thing a grown up does.
Why he started is probably curiosity. His father might have been a smoker, and he was so close to his father (before Yaldar) so he might have picked it up. Inad smoked too so it was a thing that he related with people he loved.
White never allowed it and always stopped it. But there were days where Black was beaten and too damaged for White to interfere with what Black wants to do so he’d let it slip. Similar way he acts with Black’s cursing or his anger. White always tries to never fight fire with fire.