For years I thought black jeans were just navy jeans with more confidence, and treated them the same way, bought the first pair in the right length and assumed job done. What put me right was finally trying a pair on under proper changing room light rather than the dim flattering kind, and seeing exactly where a cheap dye job goes grey at the knee within a month.
They're not the answer to everything, whatever the racks would have you believe. A big occasion that wants real dressing up still wants something with more polish than denim, however dark, and black jeans in a stiff cheap fabric can look more like school trousers than anything else, especially if the leg is too narrow and too short at once. Where they do earn their place is everywhere in between, the kind of day that needs you to look like you've made an effort without anyone being able to say what the effort was. Before buying, I check the inside of the fly and pockets for that telltale lighter denim peeking through, because that's the bit that fades first and makes the whole jean look tired after a handful of washes. I also check the rise sitting down, not just standing, since a mid rise that's flattering standing at a mirror can gape at the back the second you sit on a chair, and a waistband with no give at all will dig in by the second hour of a long lunch. Stretch content matters more here than in blue denim, because black shows every pull and strain line where pale denim hides it, so a knee that's tight going up stairs will show as a pale stress mark almost instantly. On shade, the ones that stay easiest to live with are a true flat black rather than anything with a blue or green undertone, since those fade unevenly and start looking patchy rather than softly worn in. A slightly heavier weight, nothing gauzy, holds its colour longer and resists going shiny at the thigh where friction wears it hardest.
Next time you're looking, skip straight to the inside pocket bags and the fly facing, because that hidden denim tells you more about how the whole pair will age than anything printed on the label.