Slate or smut in the undercolor was used in the early 20 s and 30 s to darken flock surfaces and undercolor. One outstanding breeder, Clarence Sibly, stated to the late Kenneth Bowles at a show in 30 s that almost all his matings had smut or slate in them. At a show, you would see feathers laying on the floor where Sibley pulled them out to avoid judges knocking his great birds down in the placings. In the early years, development of undercolor was more important than feather quality or breed type. This caused a big decline in the popularity of the breed and opened the door for the New Hampshire to gain momentum in popularity as a commercial bird.
Type is still paramount over color.
You must focus on breeding your strain on the whole. They had such a great strain of Reds, new Red fanciers would clean out their best birds each year. All they would have left were Reds that have excess black or type that left something to be desired. Yet each year they would take these left over birds breed them correctly and next season they would have more outstanding offspring for the Red fancier to choose from. The lesson here, beginner.