AVOIDING STEREOTYPES.

For that specific idea, don't have him exist solely to advance the plot for characters who aren't Black, usually through magical means. BLAMMO, success

In general, there are some pretty straightforward steps you can take that reduce huge chunks of stress and problems:

1) Go read/watch/play stuff made by Black folks. You'll get a way better idea of what non-stereotypical Black characters look like when they show up, and be supporting creators/publishers if you give them $$$ (or like, idk, comments or something if it's a free thing). I don't know your media preferences so i got no suggestions off the top of my head, but this is a thing you can google for. Like here's one thing that has some Dark Fantasy in it but idk how much you like reading or would be interested

2) Have multiple Black characters. A singular character from X group will likely be the only time X group is shown, and thus under more scrutiny than if you have more than one.

3) Spend some time reading up on Black stereotypes in general. I mean, uh, obvious, but here's a reminder just in case. Let's say that in order to avoid this character coming across as a magical negro, you decide he WON'T help anyone and will only look out for himself. Oh, and he just happens to be big and stoic and oops, now you might be running into presenting him as a big scary black dude. If you're broadly familiar with common stereotypes, you're less likely to trip into one without noticing. At the most basic, you could even just browse TvTropes for this and you'll have some idea, and I recommend reading through relevant topics in places like Writing with Color

4) Think about ways you CAN write your character(s) instead of thinking of ways you CAN'T write them. this might sound like semantics, but while it's possible to give 0 shits and write super racist stuff without caring, it's also possible to go to the other end and worry so much you get into like, "positive stereotype" territory. The same writing advice for any character still applies -- give them some Depth beyond a couple personality traits, some backstory, try to make them seem ~well-rounded~... that just applies anywhere. I mean, it's good to be aware of stereotypes or plots to avoid, but see it as a chance to explore different ideas rather than some kind of limitation to your creativity

5) In a fantasty setting, 9 times out of 10 you should skip including antiblackness. Like if you got dragons and magic and all kinds of things that don't exist in the real world, you don't NEED to include racism against Black folks (or I mean racism in general, there's ssssooo many other ways fantasy can be Dark)

Read some stuff, give it a think, good luck with your story!

Well, the Magical Negro is specifically about a minority with cool magic powers who only uses them in service of the (almost invariably white guy) hero, rather than, say, not having to work at shitty and menial jobs.

It's sort of like the Manic Pixie Dream Girl, in that the character exists solely to make things better for the hero, rather than as a person in their own right.

There are some pitfalls to be avoided in Black Folk What Do Magic, but if it's the main character, you're probably not writing the Magical Negro. ;)

Also everything Penta said.