We purchased the black dryer balls after noticing more lint on our dark clothes after drying. We had previously been using white dryer balls only.
The black dryer balls instantly fixed the lint problems on our darker clothes. We also noticed that our white dryer balls no longer had dark lint's on them as well.
For reference, we empty our dryer's lint trap after every load as well.
I would HIGHLY recommend you get a set of both white, and black dryer balls, and use in your light & dark laundry loads respectively. HIGHLY recommend.