If you can think of anything else that has happened that's unusual, every clue helps. It would help to know if your garage is insulated and if anything ever freezes, such as water, when it's below freezing weather.
So far, all we have is a very cold night, a comb that is turning black, and lethargic behavior. That points to possible light frostbite. His comb isn't white or blistered so it doesn't yet appear serious. But he may have a light case of hypothermia.
Give him heated sugar water, one teaspoon to a cup of water. For the next few hours, keep him in a warm place around 55F to 65F. Wrap him in a big, thick towel for about half an hour that has been heated in the dryer. Sit and hold him while he warms up in the heated towel. Then keep him at around 55F tonight.
Meanwhile, if you do not know how cold your garage gets, it would be a good idea to find out. Stick a thermometer in there tonight.
There is nothing wrong with providing heat if a chicken needs it. Many people do not heat their coops, and it gets way below zero. However, there may be climate considerations that make it necessary to provide heat to keep things above freezing. Not everyone can safely house their chickens in a Woods coop. But you need to figure out a way to provide heat safely and efficiently for your situation.