Why do my gardenias have black leaves?

I live in zone 5b- I brought them inside as the weather has gotten quite cold here and has been below freezing some nights. They are in a spot that is neither cold nor hot, not drafty, neither too bright nor too shady, have indirect sunlight as this is what mine preferred outdoors, but now all the new growth appears black and crispy. I also want to know if I should trim it back or re-pot it it is in a 12" dia pot and it grew so fast, it is literally falling over the pot and branches are laying on the floor. How big do these beauties grow? They were in a 4" pot when I got them and they loved me until I had to bring them inside.
Why do my gardenias have black leaves?
It sounds as though their indoor spot may be too low in humidity. Try setting it on a pebble tray with water so that the humidity in the immediate area is higher. As to size - I've seen gardenias in Texas that were easily four feet high and just as wide. Unless you have a dwarf variety, I imagine you can expect your plant to grow a LOT bigger.
Reply:http://www.bachmans.com/tipsheets/indoor...

this site suggests that the black leaves are from the plant being exposed to weather under 55 degrees....might want to read a good page on gardenias
Reply:Scale insect is the problem, the black sooty mould is a symptom. You can spray the scale, on a cool day, with a whiteoil mixture and some insecticidal soap. It's a contact thing so you'll need to drench the underside of the leaves. Spray 3 times 7 days apart and give your gardenias a feed of low nitrogen fertiliser. Something with a little extra potash would be good. This will help the leaves to toughen up a bit and make them less palatable to the scale and other sucking insects..