One thing that seems to be reoccurring in Dark Magical Girls is that their costumes tend to be much more on the revealing side than our sugary and innocent protagonists.
A mild example I've just noticed is Kanon Hatori, from Kiruminzoo.
Her outfit is for the most part closed, but that heart shaped opening is a little suspect.
Then we have Kraehe from Princess Tutu whose outfit is basically a skirt with two paddles extended upward to cover her chest.
Fate Testarossa is another mild example, but comparing her to Nanoha, we can definitely see something going on with that. People sure love that little bit of skin that shows between the skirt (or in this case leotard) and stockings. Her outfit is also much more skin-tight, which if I recall is justified because she's much more speedy than Nanoha.
Then we have the Dark Magical Girl from 6Hearts Princess who is almost entirely topless with a couple of pasties on her chest.
It would make sense if this was done as a reflection of the protagonist's purity and innocence, compared to the Dark Magical Girl who aren't as sweet.
While I wouldn't really call them evil, I feel like this has a lot to do with the trend of making female villains sexier, having them wear skimpier clothes. Black is also considered inherently sexy, and thus most female villains wear skimpy, black outfits, bonus points if leather is involved. They can also be prone to acts of seduction, whereas our protagonists would probably become nervous wrecks if they so much as spoke to someone they had interest in.
It puts me in a weird area emotionally because I'm so used to thinking of kids shows as squeaky clean, but as far as magical girl shows go, Precure tends to be the most of that, with things that come off as fanservice being pretty rare.
Sometimes we get this.
But then sometimes we get this.