In addition to not watering them and not using the Beautiful Town Ordinance, if you have other flowers, try removing them or temporarily storing them away so that your town only has black roses. Then you can make sure some of your black roses wilt the next day.
You could also find a trustworthy friend/person who doesn't have the beautiful town ordinance, drop them off in their town, wait for them to tell you they've wilted, visit, water, wait the next day and pick them up