Reply #1: Not a treatment, but a prevention strategy

[Gormy Cuss]

always water roses at the ground and don't splash water up on the leaves. Clean any fallen leaves away quickly and remove leaves from the canes as soon as there is any sign of black spot. It may not help much this year but if you are religious about these steps then maybe you won't have the problem next year. Black spot winters over and that's why the best strategy is to attack it at the first signs.

I don't know about natural remedies.

Good luck.