On the subject of real black magic spells, you should have a quick read of my other page on whether or not spells are "real" in the first place. But other than that, here is a page with some black magic spells that might be helpful for your purpose.
Bad Luck Lock
This is a pretty generic bad luck spell to put on someone when you don't have anything in particular in mind. All you'll need is:
Write a person's name on a piece of paper and fold it over once. Pierce the paper through the center to make a hole. Don't use a hole punch, it has to be pierced with something pointed. Put the hasp of the lock through the hole and lock the lock.
Brush a glob of black paint over the keyhole and on either side of the lock. Hold the lock by the hasp until it dries, while repeating the following:
By this spell,
You now are stuck.
Now bury the lock somewhere in your yard, bury the key as close to your target's home as possible. If you don't know where they live, bury it your own yard, but not near the lock.
In the Dark of Night
This is a real black magic spell specifically to bring nightmares to another person. You should cast this spell after midnight but before dawn, and be aware that it will make a bit of smoke. Have a window handy. You need:
On the paper or cloth, drawn 3 concentric circles and an x in the center. Set up the dish and charcoal on the x, and the candle outside the circles. Light the charcoal and get it smoldering, put a pinch or two of mugwort on it along with a chunk of resin. When they are both smoking, light the candle.
Watch the smoke rising from the dish and visualize it floating through space to your intended target. It brings nightmares and fear into their sleep, and picture that scene for several minutes and focus on the person.
After a few minutes, put out the charcoal and switch the dish and candle so the candle is on the x. Leave it until it fully burns down. Your victim will have bad dreams starting the next night and it will last for 3 days.