I'll tell you what I tell anyone holding a pair of black boots in a changing room: sit down, walk a few steps, then crouch like you're picking something up off the floor. If the zip pulls at the ankle or the shaft digs into your calf when you bend, that's not something a few wears will fix. I bought a pair two winters ago that felt fine standing still and turned out to gape at the top of the boot every time I sat on a train, flashing a good two inches of sock. Stand, sit, crouch, every time, before you hand over your card.
Black boots have a funny relationship with other black things. Head to toe black can look sharp or it can look like you got dressed in the dark, and the difference is usually texture rather than colour, so pair a smooth leather boot with something that has a bit of grain or weave to it rather than another flat black surface. I'd also steer away from very delicate, floaty fabrics with a chunky boot, a heavy sole under a thin slip dress tends to drag the whole outfit down rather than giving it the edge you were hoping for. As for finish, a matte or lightly worn leather ages far better than anything high shine. Patent looks lovely in the box and shows every scuff and splash mark within a fortnight. A proper matte or suede finish hides scrapes, settles into creases that look earned rather than scruffy, and forgives a bad week of weather.
How they wear depends almost entirely on what they're made of and how much you bother with them. Real leather needs feeding every few months, a decent cream or conditioner rubbed in properly, not just wiped over, or the leather dries out and cracks along the fold at the front. Suede wants a stiff brush and a spray before its first outing in the rain, not after. I've had a pair go three winters on that routine and look better for it, the leather softening rather than ageing badly, while a cheaper unconditioned pair I ignored split at the crease before Christmas. Keep them away from radiators when they're wet, they'll dry stiff and shrink slightly at the seams if you rush them.
They're not the answer to everything, mind. If you're after something for standing all day on concrete, a stiff new boot with no give in the sole will leave your feet aching by lunchtime, no matter how good they look. And if your calves are on the fuller side, a tight shaft boot will dig in and leave a mark that lasts the whole evening, so check the width as carefully as the length. Reach for black boots when the forecast's turned, when you need something that goes with nearly everything else in your bag of clothes without a second thought, and when you'd rather spend five minutes on your feet than five minutes deciding what to wear.