Black sandals get called boring by people who've only ever owned a cheap pair, and I understand why, but a good pair earns its reputation for the opposite reason. They're boring the way a good knife in the kitchen is boring. You stop noticing it's there, which is the whole point. I've got a pair that's done three summers, a wedding, and one memorable walk back from a car park on gravel in the dark, and I still reach for them over anything newer.
If you've got none in the rotation, start with a simple strappy flat or a low block heel, something with one buckle or a slip on strap, nothing fussy, because you want it to go under trousers and under a dress without an argument. The difference between a good one and a disappointing one shows up fast once you've worn it an afternoon. Cheap sandals use a stiff, shiny strap that doesn't soften, so it rubs the same spot on your foot every single time, and the sole is flat plastic with no give, so by hour three your feet feel like they've been standing on a kitchen worktop. A decent pair has a strap with a bit of a matte finish and some flex to it, stitching that's even rather than glued and bubbling at the edges, and a footbed with the faintest bit of padding so your weight isn't landing straight on hard ground. Hardware matters more than people admit, a buckle that's already a bit loose in the shop will be properly loose by September. Press the sole with your thumb in the shop, if it doesn't give at all, put it back.
For pairing, black sandals are the one shoe that doesn't ask your outfit any questions. Denim, a plain linen dress, wide leg trousers that skim the top of the foot, even a slightly dressy top with shorts, they sit underneath all of it without competing for attention. I wear mine with pale colours especially, because black at the foot does something clever and grounds a washed out outfit without heavying it up. Where they properly shine is anything from late spring through to early autumn, a casual lunch, a long day of errands in the heat, an evening that starts at a barbecue and ends somewhere with better lighting, they don't care, they just keep going. Wet pavement is the one place I'd be careful, a flat leather sole with no tread gets properly skatey on anything damp, so check the sole has some texture if you're anywhere near water.
Reach for them on the days you haven't decided what the outfit's doing yet, and need the shoe sorted before the rest of you is.