How do you remove black marks from car exterior?

The first question is what caused the black marks?

Rubber tires leaned agains or piled on the car? Usually a carefull rub down with a cleaner like Acrosol or brake clean wil take that of. CAUTION these chemicals can damage some paints, BE gentle.

Tree sap, good car wash soap will clean this.

Etching in the paint because moss has grown in the cracks and cranies. Good soap followed by a light cut polish will make for the best result.