The mistake is treating a black co-ord as a safe default and grabbing the first one without checking the two pieces actually match. Black is not one colour on the high street. A top dyed in one batch and trousers or a skirt dyed in another can sit next to each other looking faintly wrong, one leaning brown, one leaning blue, and you only notice in daylight, usually once, outside the shop, too late to do anything about it.
Worn as a set, a good black co-ord holds up well, but the two pieces often age at different speeds if you wash them differently, which is how you end up with a top that's gone slightly grey while the trousers stay deep and dark. Wash them together, same cycle, same temperature, every time, even once you start wearing them separately with other things, because that's what keeps the match true for longer. A cool wash and hanging to dry does more for black than any fabric claim on the label. Cheap black fades fast under a hot dryer, and once it's gone patchy there's no getting it back.
Once you're not wearing it as a strict set, the separates do a lot of quiet work. The top goes under a denim jacket with pale blue jeans for something that needed no thought on a weekday morning. The trousers or the skirt pair with a plain white or cream top and trainers for anything daytime, or a simple jumper when it's cold. A longer line skirt from a co-ord works well with an opaque layer underneath and flat boots, and the proportions tend to be kinder than trying to match a skirt bought separately, since the two were clearly cut to sit together from the start.
Matte black is easier to live with than anything with a sheen, which shows every crease and catches the light in a way that reads more evening than everyday. A slightly soft, almost charcoal black wears better day to day than a stark, inky black, which can look severe under office lighting. In the shop, check the fabric against itself in both pieces under a window rather than the shop's own lighting, check the hem and cuff length separately since co-ords are cut as a pair but don't always fit the same way on every body, and check the waistband or neckline doesn't need its own fix before you've even left the till.
Reach for a black co-ord on the days you want to look like you made an effort without having made any decisions at all, work, travel, or anywhere you want to blend in smartly and get on with the day.