hapynhim
Zuzu and CindiKS are correct that when you see those black spots on the foliage of a rose, it indicates the rose disease of black spot. There is one exception that comes to my mind. If your grandmother's rose is what we call an "old garden rose" or ogr that naturally sheds its leaves as part of the process of going dormant for the winter months, the leaves will often show black spots and may turn yellow as the rose prepares to abandon the leaves.
Dormancy in roses is triggered by cooler temperatures and shorter daylight hours. If the rose is going dormant, those black spots are just an indicator that the rose is no longer providing nutrients to the foliage and is hoarding all of the nutrients in its root zone.
You may want to put a layer of mulch over the area you have cleared of mint to block light from any mint you may have left behind to inhibit the growth of new mint plants. In spring, you should replace that mulch with new mulch. If the rose does have black spot, the spores over winter in the old mulch. Replacing that mulch will help your rose stay healthier.
Good luck with your rose.
Smiles,
Lyn
