The first thing I do with any black summer piece is hold it up to the window. Not the mirror, the window. Cheap black fabric goes slightly green or grey in daylight, and you only ever find that out once you're outside a shop and not before. A good black stays black in full sun. That's the whole test, and it takes about four seconds.
In the shop I'm checking the weight of the fabric before anything else, because black absorbs heat in a way that pale colours just don't, and a heavy black dress in July is a decision you'll regret by lunchtime. Linen blends, cotton poplin, anything with a bit of drape and a weave you can see light through when you hold it up, those are the ones that survive a whole day outside. I check the lining too, because a lot of the cheaper black dresses have a stiff polyester lining sewn in that turns the whole thing into a sauna, and you can't tell that from the outside, you have to put a hand inside the garment and feel it. Seams matter as well, since black shows pulled thread and puckering more than any other colour, it hides stains but it has nowhere to hide bad stitching.
Where it falls short is obvious once you've worn it on a proper hot day, the kind where you're on your feet for hours, because even the best black fabric runs warmer than the same cut in white or sage or that washed out pink everyone had a few summers back. I wouldn't reach for black for a midday garden thing in August, or anything where you're going to be stood in direct sun for a long stretch, it's just not the kind thing to do to yourself. It's also the wrong choice if you want a dress to photograph with any texture, because black flattens detail, pleats and broderie and any nice stitching basically vanish into it from a few feet away. Good for the shape of you, not so good for showing off the work that went into making it.
As for what it clashes with, mixing several different blacks in one outfit is the fastest way to look like you got dressed in the dark, which, fair enough, you probably did if it was five in the morning and you were catching a flight. A faded black bag against a crisp black dress reads as mismatched rather than layered, so I try to keep the blacks in one outfit roughly the same depth, and I steer well away from black paired with very shiny gold hardware in full sun, it just looks hot and heavy together. Would I still want to be in this at four in the afternoon with no shade in sight?
