There are several things that can cause black foliage. One is a sooty mold that grows when the plant is infected with aphid. The honey dew or poop left by the aphid grows mold. This can be washed off with warm soapy water and the aphid removed with strong jets of water as well. A horticultural oil sprayed on the plant will help control aphid. The other  cause of black leaves is high temperature and if your plant is indoors near a bright window it may be that the gardenia was hit with too much hot sun through the glass. Be sure the plant has plenty of humidity by misting it often or setting it on a tray of pebbles and water. Adding  a slow  release  plant food like Osmocote every spring is recommended as potted plants need fertilizer. The drop of buds could also be related to high temp or the plant drying out. You can save this plant so don't give up. Clean leaves and lots of humidity with shade from hot sun are the secrets of success with gardenias. Keep growing, Marianne Binetti