The Blue Rose says that they will never make him in their town by making two red roses breed. He does this by wilting all the red roses in the players town that were made as hybrids. He also destroys flowers that can make them if they are able to hybrid.
The Blue Rose is the ultimate flower in Animal Crossing. Like the Purple Pansy, it must be made with 2 hybrid flowers that are Red. In its case, its hybrid red roses. The blue rose was picked over the purple pansy because of it being notable. This flower and the Golden Rose, another character in the series, are considered the "Ultimate" Flowers in Animal Crossing.