The best solution for black spot is to get roses that are so vigorous that they aren’t susceptible.  There are plenty out there, but if you get a rose that succumbs to black spot that you just can’t give up on (and who hasn’t?) then there are several things you can do :

1. Keep water off the leaves.  Water at the roots and not overhead.
2. Water deeper, but less often.
3. Prune to increase air flow to the inside of the bush and remove the lowest limbs so that air can flow from underneath.
4. Improve the soil with compost, mulch, earthworm casting and biostimulants (like compost tea, molasses, fish water, etc.).
5. Spray the leaves early in the day with a baking soda solution of 4 teaspoons of baking and one teaspoon of non-phosphate soap (not detergent!) to a gallon of water.  I like to use Dr. Bronner’s peppermint soap.  You could also use vegetable oil or a summer weight horticultural oil instead of the soap.  This is a great fungicide for use on the leaves and not for use on the soil or it can harm other organisms that you need for a healthy soil.
6. Keep diseased leaves cleaned up but don’t put them in the compost pile or you may just spread your problem.
7. Keep the plants as healthy as you can by using a compost tea as a foliar feed.  Instead of making my own, I usually just buy Garret Juice and Liquid Seaweed, but I make my own when I can’t afford those.
8. If the problem is not too bad, just live with it.  If the problem is really bad, get rid of the rose.  That is just not the place for that particular rose.