The first thing I do with a black bra is hold it up to the window, because that tells me more than the label ever will. A true opaque black, the kind that doesn't show a hint of nipple or seam through a white shirt, is rarer than it should be given how many black bras exist. A lot of them are black in name only, thin enough that under certain lights you can see exactly where the padding stops and the lace starts, which defeats the entire point of buying one in the first place.
Where black actually lets you down is under pale or sheer fabrics, oddly enough, the same job it's meant to solve. A deep black bra under a very light cream blouse can read as a dark shadow rather than disappear the way it would under navy or grey, so if your top half is mostly white linen in summer, a nude or soft grey bra will do a quieter job. Black also shows wear faster than any other colour, which sounds backwards but isn't: every wash pulls it a shade towards charcoal, the lace yellows at the edges before the main fabric does, and underwire casings go slightly grey while the rest still looks fine. Cotton blends hold their colour longest, microfibre fades quickest, and anything with a high percentage of elastane in the band will lose its grip well before the colour gives up, so the band going slack is usually the real sign to retire it, not the shade. Hand washing in cold water and leaving it flat to dry keeps the elastic honest for longer, a tumble dryer will finish a good bra off in about four goes. I keep two or three in rotation rather than wearing one until it's thin, because the recovery time between wears matters more than people expect, elastic needs a rest as much as you do. Black earns its place year round, under dark knitwear in winter and under anything remotely see through in summer, it's the one I reach for on days I haven't planned an outfit and just need something that won't argue with whatever's clean.
The mistake is buying several in one go before you've actually worn the first one for a full day, because sizing varies enough between styles that you can end up with three identical mistakes instead of one.