Some homemade fungicides are:
(1) steeping 16 bags of chamomile tea or 2 cups of dried chamomile flowers in 2 quarts of water for 20 minutes. Strain the tea or remove the tea bags and spray onto affected plants.
(2) A combination of 2 crushed uncoated aspirin dissolved in 1 quart of water also makes an all purpose fungicidal spray.
(3) Powdery mildew and black spot can both be treated with a mixture of 2 teaspoons of baking soda, one-half teaspoon of liquid soap or oil soap and 2 quarts of water. Be sure to test the soap solution on the plant as some plants do not take well to these solutions.
(4) A spray comprised of 1 part milk and 9 parts water is another homemade fungicide used a lot for powdery mildew but some folks have said it works on black spot as well over time. You can increase the water to milk proportions to make it a stronger solution if needed but I've never know anyone to use more than a 20% milk solution. Some folks add some plain yogurt to the mixture to strengthen it up some.
Of course, following good practices with the roses to prevent black spot is important so watering beneath plant leave, keeping the garden bed area clean, and having good air circulation are equally important.
It took me 3 years to eliminate the black spot on one of my miniature roses but it looks great now