Certified GKH Gardening Expert
Four months should be enough time to get established. Black spot doesn't usually kill the rose bush but it helps to pick up all the affected leaves that fall to the ground and destroy them. That will help avoid it next spring. Good air circulation and full sun also help prevent black spot disease.
If you have a hybrid tea rose, they generally need to be on a prevention spraying program in the spring because they are more susceptible. Many shrub roses and "easy" roses like Knock Outs don't need the spray. Here is more info:
https://www.gardeningknowhow.com/ornamental/flowers/roses/black-spot-roses.htm