Answer: Unfortunately, based on your description I am not certain what is happening to your gardenia. It might be sooty mold, a fungal infection that someimes accompanies an infestation of whiteflies, aphids, or other sucking pests. Winter would probably have killed off the whiteflies or aphids, and then the fungal problem would lessen on its own. Whether or not it would recur would depend on the weather this year.
Since I can't say for sure long distance, I would suggest you consult with your local county extension for a specific diagnosis, and then based on knowing that, they can help you determine what to do for it. If it is something that requires a chemical treatment, they will have the most up to date recommendations on what to use and how/when is best to apply it. They might appreciate a sample (freshly cut and enclosed in a plastic bag) and some photos both close-up and of the plant overall to help in their diagnosis.
Pruning is usually done, if necessary, right after its main blooming in the spring. I would not recommend pruning however until you have identified the cause of the black foliage and developed a care program.
Generally speaking if it has done well for you until now you would not want to change your care routine for it. Blooming would not be dependent on pruning but instead related to the overall health of the plant. Drought, disease, pests can all reduce the plant's strength with reduced blooming as a result.
Good luck with your gardenia!
Q&A Library Searching Tips