It depends on how deep the scratch. I use Malm's car 2 step wax. I have used it for 30 years this year. It was made specifically for black cars/paint but can be used on anything. The first step is the fine polishing cleaner then the pure carnauba wax.
I've had amazing results with this product. It can be purchased online if interested. I bought it originally for my new 1984 Peugeot 505 STI (black) and it started to get spiderweb scratches in the paint from regular washing. The two step Malm's wax made it shine like glass. It's not cheap but works miracles.
MM.