The question I get most is whether dark denim actually goes with everything, and the honest answer is almost, not quite. It's the closest jeans get to foolproof, but I've learned the hard way that almost isn't a licence to stop thinking.
Where it falls apart is pairing dark wash with anything else trying to be moody and heavy at the same time, a chunky black boot and a stiff dark denim can end up looking like you got dressed in a cave with no mirror. It also doesn't love being next to washed out, distressed denim on another piece, two different blues fighting for attention is never a good look, and a dark wash paired with a top that's too casual, like a baggy logo tee, can tip the whole thing into looking like you've given up rather than dressed down on purpose. It wants something with a bit of structure near it, a proper shirt, a blazer, even a simple knit, to earn the smartness it's capable of.
The difference between a pair that lasts and one that's bagged out by October is mostly in the weight of the denim and what's holding it together. A good dark pair has enough weight that it doesn't go thin and shiny at the knee after a few months, you can usually tell by pinching the fabric, if it feels like it could double as a tea towel, walk away. The dye matters too, a proper dark indigo that's been through a decent finishing process will fade slowly and evenly, going a touch softer blue with every wash rather than patchy or grey at the seams. Check the stitching on the pockets and the fly, uneven or loose threads there are usually the first thing to go. Rivets and buttons should feel solid, not like they'll pop the first time you bend to pick something up off the floor. The ones that have properly let me down were always the ones that felt a bit too light and a bit too cheap in the shop, and I knew it, and bought them anyway.
Where dark denim doesn't help you is anywhere genuinely formal, it reads smart casual at best, never office presentation or anything black tie adjacent, and it's not the one to reach for in real heat, the colour holds warmth more than a lighter wash does, so on a proper hot day it's not doing you any favours.
For most of the year, most situations, a good dark pair is the one I reach for without having to think about it, and that's worth quite a lot.