Just how often to water any plant depends on many factors starting with the soils ability to retain moisture and release that moisture to the plant. Clay soils tend to hold large amounts of moisture and not release that to the plants while sands tend to be too well drained and not retain any moisture which is why lots of organic matter in either one will help the soil to be evenly moist and well drained and can make that moisture available to the plants.
There are some people that rigidly maintain that wetting a plants foliage will create conditions that result in plant diseases while I have seen plants get diseases when the foliage is not wetted while watering, except by rain. I have also seen plants no get any disease even after the foliage is sprayed while watering, and some believe that spraying the foliage washes potential disease pathogens off so they cannot develop.
I have seen Black Spot on newly purchased roses just in from a nursery and I control that with sprays of 1 teaspoon of baking soda mixed in 1 quart of water or a 50/50 mix of fat free milk and water, sprayed every 5 to 7 days.

This problem will go away by itself when the roses grow bigger. My knockout roses also had this problem, now they are all ok.

All of that extra watering might be a contributing factor to your problem. Black spot is one of the diseases that MUST have moisture in order for the propagules to germinate and to spread.

It is powdery mildew that requires no water, and can be rinsed off foliage before the fungal spores germinate, without creating an environment for infection.

You should know that Knock Out roses are quite drought tolerant and do best if not over watered. Every other day is too often for almost anything, other than a very thirsty vegetable garden. Frequent, shallow watering cycles promote shallow, limited root systems.

Even in times of serious drought (and believe me, I have lived through that), fewer but deeper watering cycles are best for the plants.