USRT's 256mm slotted-only rear rotors are black zinc coated for long-term durability. Slots constantly skim/level both brake pad surface for maximum coverage, gripping, and stopping power. They vent pad material gasses that would otherwise push the pads away. Also, look closely and see that the center hat features a radial array of venting/lightening holes.
These are premium parts. Black ZINC plating lasts far longer than inferior but common black oxide coating. Coloring on contact surface is worn away within about 10 miles. All the rest lasts the lifetime of the part, keeping it beauful and "ready to fight".
Applications:
VW MK4 Golf 20th AE, Golf 337, Golf R32
Audi MK1 TT 225 Quattro, TT 3.2 V6