There's a particular stretch from late September through to about March when black trousers stop being an option and start being the only thing I reach for, because everything else in the wash is either too summery or covered in something I'd rather not examine too closely. Work do, funeral, parents' evening, the three occasions that all somehow land in the same fortnight. One pair, endless reasons to wear it.
A decent pair earns its stripes slowly. The knees go first if the fabric's thin, that slight shine appearing after a few sits on a train or a desk, so check the care label before you chuck them in a hot wash and watch a crisp pair go soft and baggy in one cycle. Cold wash, hang to dry, and if they've got any wool in them a steam rather than an iron will save you scorching a line down the front crease that never quite drops out again. The honest limit is that black trousers are a terrible choice for anything genuinely hot, they absorb heat like nobody's business and show every bit of chalk, lint or dog hair going, which is its own small tragedy if you've got a dark haired dog. They're also the wrong call when you want to look relaxed rather than assembled, there's a brisk, slightly formal note to black that doesn't do casual as softly as navy or a washed grey would. On sizing, go by your waist and hips rather than a size on the label, because black trouser sizing seems to wander more than any other colour, I've no scientific proof of this, just a drawer of evidence. Watch for gaping at the waistband when you sit, and check the rise doesn't dig in when you're at a desk for hours, since a trouser that's fine standing in a changing room can turn into a different, more hostile garment by hour three. Length matters more than people admit, a half inch too long and they drag at the heel and fray within a season, too short and they read as a mistake rather than a choice. As for what they clash with, avoid pairing them with anything else trying equally hard to be formal, a very structured blazer on top of very sharp trousers can tip into looking like a uniform rather than an outfit, and stiff, shiny black tights with matte black trousers do something slightly odd to the eye, like two different blacks arguing.
Buy one proper pair, look after it properly, and it'll quietly outlast three trend buys you forgot you owned.