Will the surface of a black granite kitchen sink scratch？

That completely depends on whether you buy a PMMA-type "real granite" sink or a UPR-type "imitation granite" sink:

High-quality best granite sinks are made from PMMA resin (20%) and quartz sand (80%) through a thermosetting process. Their surface hardness is close to Mohs 7, making them extremely difficult to scratch with metal tools. If you see scratches, they are metal residue adhering to the surface (like pencil marks), which can be removed by scrubbing vigorously with a scouring pad and detergent; it doesn't mean the sink itself is damaged.

Inferior quartz sinks, in order to reduce costs, are made from UPR resin (over 30%) and quartz sand (less than 70%) using a room-temperature process. Their surface hardness is low, only Mohs 3. A single touch with a steel wool pad or knife (usually Mohs 4) will leave a permanent, deep, and irreparable groove.

In general, only quartz sinks made from UPR resin and quartz sand using a room-temperature process are easily scratched. PMMA granite sinks, on the other hand, are unlikely to be scratched by household ironware.

When choosing a the best granite sink, prioritize PMMA granite sinks, such as the NAITRON granite sink from China, which has a Mohs hardness of 7, second only to diamond.