I avoided black dresses for years. I thought they were the choice you made when you'd given up thinking about it, the fashion equivalent of ordering the same thing every time because you can't be bothered reading the menu. What changed was a difficult week last winter, three events in a row where nothing else felt right, and I finally bought one properly instead of pulling the oldest thing in the cupboard off a hanger. It wasn't boring at all. It was just doing its job without asking for attention.
Once I started looking properly, I noticed how much separates a dress that lasts from one that bags out by October. Pull the fabric gently between your fingers in the shop, away from the mirror, and see if it springs back or stays stretched. Check the lining, not just whether there is one, but whether it moves with the outer fabric or drags against it when you walk. Look at the zip, run it up and down twice, because a sticky zip on day one is a broken one by month three. Hold the hem up to the light if you can, since cheap black fabric sometimes goes slightly see through under shop lighting that flatters everything, and you want to know that before you're stood under a brighter bulb somewhere less forgiving. Seams matter too, run a thumb along the inside and feel for anything unfinished or already fraying.
Black dresses get tripped up by small, avoidable things. Sheer black tights with any shine to them can read as slightly dated against a flat matte dress, and a very glossy patent bag next to a soft, textured fabric can make the whole outfit look like it's arguing with itself. Two different blacks worn together, a dress and a cardigan from different decades of your wardrobe, will often clash more than you'd expect, because black ages differently depending on the dye and the fabric, going slightly green or slightly brown over time. The easiest shades to live with are the ones with a bit of texture, a crepe, a fine rib, a slight slub, rather than a dead flat black that shows every mark and every stray thread. Matte finishes forgive far more than anything with a sheen.
The good ones have a weight to them, a swing when you move, that makes you stand up a bit straighter without thinking about it.