I believe we are in zone 8.
If it is a hardy hibiscus, than it needs no winter protection. If it is a tropical hibiscus, it is iffy if you can winter it outside in your area, but it is worth a shot. Mulch the roots heavily and wrap the plant in chicken wire (or something similar). Fill this with leaves or straw. Then wrap with burlap, leave the top open. Remove the wrap and mulch in early spring.
Plant was outside all summer with many blooms and beautiful color. . . . now flowers have weak color. Do I need to fertilize? And if so, what do I feed it?
The issue is probably more about how much sunlight the plant is getting. It can be very difficult in the winter to get a hibiscus enough sunlight to both grow and flower, particularly if you have brought it indoors. Winter sunlight is just not as strong as summer sunlight. I would recommend trying a plant light on it. That may help, but you still may have to wait for spring for the best blooms to return.
What should I feed hibiscus now, early December? It is indoors and still flowers, but needs help. It has never been fertilized.
I would wait until spring to fertilize. No matter what you do in the winter, your hibiscus is going to look a bit ragged. They are not an indoor plant and being indoors is just good enough to get them to survive. If you give them fertilizer in the winter, it could be too much and could hurt them.
Why are the leaves and stems of my hibiscus turning black?
It sounds like sooty mold. This article will help:
https://www.gardeningknowhow.com/plant-problems/disease/how-to-get-rid-of-sooty-mold.htm
I have an indoor hibiscus plant. Are the leaves toxic if eaten by an animal? I have a pot belly pig that I have had to pull away from the plant at least twice. I'm a little concerned if he gets a few leaves. I have to keep the plant inside because of winter cold.
It is hard to say for certain as applies to pigs, but hibiscus is not toxic to humans. Pigs are very close to humans from a biological standpoint (even at one time being used for heart transplants) so we suspect that the hibiscus would not be toxic to pigs either. But, just in case, keep an eye on him and try to keep him away from the plant.
I have about 10 Hibiscus that I would like to save over the winter but do not have the room to care for them inside. Can I remove some or all of the root ball soil and then let them dry out in a dark place until spring? Can a Hibiscus survive something like that?
Yes, you can do this. Make sure the location is cool (not cold) and that you dip the roots in water once every 4-5 weeks while you are storing it. It will look dead during this time, but in the spring, when it is planted again,it should come back fine.
We are in Houston. It will be getting down to 25 degrees F tonight, with the next day 50 degrees F and a low of 40 degrees F. What should I do for the one night? Do I need to do anything for my hibiscus tomorrow too? They are not in pots but in the ground, so uprooting is not an option.
Cover the plant with a sheet or blanket. Make sure to remove it first thing in the morning. This will help insulate it from the cold.