Hi Susan, I'm not sure exactly what is ailing your mandevilla. Most diseases are specific to their host so doubt it is the same as black spot on roses. Could be fungal or bacterial. A common fungal one is cercospora. Check out this website for pictures to see if it looks like yours.
http://njaes.rutgers.edu/pubs/fipmnotes/2007/fipmnotes17-03.pdf It's a disease that is more common with overhead watering and poor air circulation. You may want to contact the place where you purchased it. Usually removal of infected leaves and keeping leaves dry will help to curb disease. fungicides such as Daconil can be needed if the disease continues. Be sure to read and follow all label directions. Hope this helps. Sandy