I have used both of these with good reults over the years.
1 teaspoon of Baking Soda mixed in 1 quart of water or a 50/50 mix of fat free milk and water either one sprayed every 5 to 7 days on infected plants. Clean up infected leaves and canes before spraying.

Yes, there are variations on kimmsr's recipe of baking soda, some use 1 more tsp for really bad blackspot. I used to make in a qt sprayer 1 tsp baking soda and 1 tsp veg oil as a sticker and you have to spray a lot, in morning or evening. On succulents not sure if it will work, when I got an infection in Seattle I coated fungus in veg oil and that smothered it, still have black on an aloe to this day.

Dan

Thanks kimmsr and dan for your replies.
I will try both of these recipes.
As a test on few of my aloe plants, I will try to mix in neem oil and soap along with baking soda and monitor the progress. thx, -j

"Black Spot" is a disease of roses, although it could potentially also infect apples since they are the same family of plants. Often other plants do get some black spots but those are caused by many other bacteria or viruses, or mechanical damage, and I've not seen that Baking Soda is very effective in controlling those. For example, Maples (Acer species) every year get Black Tar Spot which is not controlled by Baking Soda or much opf anything else. But that also does not do much harm to the infected trees.

The baking soda spray will work, and works better with an oily substance added to help it stick. I add a drop of liquid soap, too, to help lubricate it.

BUT - you have to spray BEFORE black spot hits so start early in the spring as the roses are growing. Baking soda will not "kill" black spot once it starts - it is purely a preventative.