The black film is Sooty Mold - it grows on the juices excreted by a sucking insect - usually either whitefly, scale, or aphids. The problem isn't the sooty mold, although when it's thick enough it will prevent the leaf from doing it's job, and leaves may drop as a result....but the situation will only improve if the insect problem is under control. If the plant isn't too large you can either spray with a hard stream of water to remove the sooty mold, or wipe with a mix of insecticidal soap and water. But first, figure out which insect is secreting the "honey dew" (excess plant juices) that the mold is growing on, and treat accordingly.

Excellent advice! Thank you very much.

If it's gardenia, it is the whitefly, more than likely.

These pests can be very stubborn to control, and seem to have a real love of gardenias! Insecticidal soap applications directed to the under side of the foliage where the whitefly hang out (in all life stages) can be repeated throughout the season. You might also want to try Neem oil, which has strong anti-feedant properties.

Once you get the pests (and I am only assuming that it is whitefly) under your control, the black sooty mold will begin to flake off. You might want to look for those other insects, too. Scale insects can sometimes be a pest of gardenias.

Marathon granules work great on whitefly, if you don't mind a chemical pesticide.