I spent ages thinking black jeans were just blue jeans in a different mood, so I kept buying them the same way, and ended up with a drawer full of pairs that looked faded and a bit sorry for themselves after six washes. What sorted it was realising black jeans are their own category with their own rules, mostly about the dye wearing off before the shape does.
They're not the answer to everything, whatever the posters would have you believe. On a bright day they can look heavier than the weather deserves, and if your day involves sitting cross legged on a floor or a long car journey, a stiffer black denim will crease across the front of the thigh in a way softer washes don't. They're also not forgiving of a bad length, a dark jean puddling over your trainer reads scruffy in a way a lighter wash gets away with. Before you buy, check the inside of the waistband for a proper facing rather than just folded fabric, that's usually the bit that goes thin first. Check the pocket bags too, cheap ones are cut too small and gape slightly at the hip even on a good fit elsewhere, you'll see it from the side in the mirror, not the front. Give the fabric a stretch and let go, if it doesn't bounce back close to where it started you'll have knees by lunchtime. And look at the rise against your own waist rather than the model's, a jean that sits perfectly on a longer torso can dig in or gap at the back on a shorter one, no size fixes that, only the actual cut does.
For colour, go for a true flat black over anything with a grey or blue undertone, the undertones are what show up as faded first and look less like a style choice and more like an accident. A black with a touch of stretch holds its depth better through washing than a rigid cotton, which tends to go patchy at the seat and thighs exactly where you'd rather it didn't. Satin finishes and coated denims look brilliant on the hanger and less brilliant after a few sits down, the coating cracks along the crease lines. Matte, plain black, decent weight, is the one that still looks like black jeans a year in rather than a memory of them.
Next time you're looking, skip straight to the inside of the waistband and the pocket bags before you even try them on, that tells you more about how it'll age than the colour ever will.