One of my flashlights that has a black anodized aluminum housing has a couple of scratches on it, all of the way through the anodized coating.
How can I cover up the scratches so that its completely black again?
How to touch up scratches on a black anodized aluminum flashlight?
Try using black nail polish on your flashlight 🔦. But make sure it is a good quality nail polish and not something from the Dollar Store. People have used nail polish to remove scratches on cars for many years now.
Some people may only know nail polish as something women put on their nails as a form of an accessory. However, nail polish has a lot of uses, and one of these uses includes being a great scratch remover. As you can tell with the other DIY fixes, you will need to buff and sand. However, with nail polish, you will just need to clean the area before applying it. Nail polish helps cover up the scratches instead of doing a lot of legwork to “remove” the scratch. Furthermore, nail polish comes in a variety of colors, so it’ll be easier to match a color with your paint. This makes it a really effective way to remove scratches on your car paint. So, you should look for the closest color to your car paint that you can find. Apply the polish as evenly as possible so the scratch will no longer be seen. If you still see the scratch afterward, you can just apply another coat of nail polish after it has dried. - Nail Polish
"How can I [make] it completely black?"
Cover it with something black.
Drawing on it with a black marker will work, but that might rub off after serious use. You could use black spray paint, but that requires etching into the rest of the anodized part. Black electrical tape would be great, and makes it a little more grippy when wet, but gets gross after a while.
I have a small can of Bar-B-Q-Black, a Rustoleum product that we have used on scratches or dings on black metal objects, such as a wood-burning stove or a iron garden bench. This particular can says it resists heat up to 1,000 degrees F, which performance limit you are unlikely to need.
If you Google Bar-B-Q-Black, Rustoleum you will find an array of products at an array of prices at many places. It may be more expensive than nail polish (see answer of Ken Graham) but not necessarily.