All black almost equal chuuni, kinda reminds me of another manga that all of the mc team like the demon lord daughter, a dragon girl that cant transform into a dragon yet etc, wore all black or some bdsm style clothes that well.. Black colored.
Eh... I'd say the chuuni factor resides in more than the clothes' color. To begin with, for a story to really be chuuni, it needs to gun for the rule of cool and some clothes are not compatible with this vision. For example, bdsm suits don't strike me as cool, but weird and off-putting. Or maybe you were talking about spandex suits, which are rather generic (if suggestive) by themselves and need some flourish to be pushed into 'cool territory'.
What's more, a chuuni story often follows an average dude (or chick, it depends) stumbling into a violent magical conspiracy, so their initial looks are often whatever. However, for the protagonists who do get an interesting outfit, it mostly gravitate around a few key points: long flowing coats (or waist cape) and jewelery with weird symbols. Maybe with an eyepatch of power or some bandages, if the author pushes his luck. Or at least, that's what I observed. All in all, a kind of stylish soft punk-like aesthetic.
On the other side, it's most often the antagonists who are the biggest edgelords in those stories, and thus carry the most chuuni looks. I'll only speak of the most renown chuunige here, but looking at Dies Irae's Longinus Dreizehn Orden and Fate/Stay Night's Kotomine Kirei, Matou Zouken and Gilgamesh should give you an idea: they are more classy and confident in their style, even if their personalities are more varied. So, quite amusingly, a truly 'chuuni'-styled character is more likely to incorporate some formal elements in their attires if they are an antagonist in the story.