If you're interested in black magic then I suggest you first understand the difference between it and white. White magic is generally gentler and focuses on blessing and changing things through blessings.
Black magic focuses on direct manipulation and doesn't place strong emphasis on worrying about the damage caused.
This is a very reductive way to look at it, but you'll grow to understand things better as you improve and progress.
There arent set black magic rituals, it's more a style of magic.
Power, in magic, comes from you and not the ritual. Remember, your mind is what's really casting the spell. Not the ritual tools.
I suggest you work a little with evocations, lillith is a nice start point: very gentle aura.
If you're unsure about evocation techniques then there's plenty of info out there to get you started.
Everyone has different experiences with different entities, what may seem gentle for you could very well seem 'harsh'..for lack of a better word..to someone else.
The only difference between "white" and "black" magick, is the intent.
That's a good point, though I'd have to disagree on the point about the only difference being intent.
You could have a black magic ritual that protects you, you could also do this with white magic. At the end of the day, both have the same intent but are based in different types of magic.
In this case, Id say the difference would be the type of energy used and any spirits/demons who may have been evoked.
We see these terms get tossed around all the time. If you want to call it black or white, and that works well for you, so be it. But many do not see magic like that. A lot view it as the will of the caster determines what the magic will be and the overall outcome. While magic itself is neutral.
You can work with spirits and demons regardless of what type of magic you're working. They aren't just associated with "black magic." I've worked with them for both purposes, positive and negative. They don't need to be called upon for a specific one.
Anyway, back to the original question, you can make your own, like Valeshka said. Look into curses or hexes, if you're looking to give someone a bad day. They're usuaully the most commonly discussed forms when talking about "black magic."