I don't know if your ''black bugs'' are the problem, or casual hitch hikers, rather than causal critters.
Too dry between watering can cause dry patches on leaves. Too hot, too much sun, and many other environmental issues can cause dry patches on the leaves.
Think about your horticulture, perhaps there is a problem there.
Okay, assuming that you are right and the bugs are the problem. Soapy sprays work well if you have a small collection of only a few plants and the time to repeat the process several times a week. These solutions work by mechanically washing away the pests. Most formulas are a drop or 2 of liquid soap in a pint ot quart of water. Some versions have you add some portion of denatured, rubbing or other inexpensive types of alcohol. Other formulas skip the alcohol. The soap and alcohol serve as wetting agents, assisting in penetrating the wax or oily layers some insects surround themselves in. When you spray with soapy water, you must repeat the process at an interval shorter than the time it takes for the pest's eggs to hatch, mature and begin laying eggs again. So for most bugs, at least once a week. Do this a minimum of 3 to 5 times. Continue at least 2 sprays after the last time you saw an insect. This will make sure you got the larva from any eggs that hatched. Stopping too soon is the main reason these "home remedies" fail.
People with more than a few trees often resort to chemicals because with these sprays, if you don't spray every surface, and miss a single female bug, it will be around to lay eggs. With a large collection there is not enough time to be that meticulous. But with small collections these sprays can work well.
