I've never had a black eye, but a few years ago I was in a car accident that turned my entire ear dark purple.
Ideally, you want to use an ice pack, but if you don't have one a bag of frozen vegetables, ice cubes (or whatever else you have in the freezer) works just as well.
Whatever you use, wrap it up in a dish towel to cover any sharp edges and make the temperature a little more bearable for your skin.
Apply the cold compress to the bruise for 10 minutes, then take it off for 15 to rest.
Repeat the cycle several times within the first 2 to 3 hours after the injury and try to keep your head elevated as much as possible to reduce pressure.
After 24 to 48 hours, you'll want to switch from cold to a warm compress to help increase blood flow to the area, which will speed up the healing process.
You can make this super simple by using warm water to dampen a towel and holding it over the injury until it's cool.
After the discoloration that comes along with a bruise has already shown up, it's a little harder to deal with, especially if you weren't able to prevent any of the damage by treating it early.
It's one thing if you actually have a good story, but most black eyes happen in ways that we'd probably rather forget.
There really are no miracle cures that instantly make bruises disappear, but there are a few things you can do both before and after they appear to mitigate the damage and save some face (yes, pun intended).