I avoided navy bags for years on the grounds that they were a sort of compromise colour, neither here nor there, the sensible shoe of the handbag world. What changed was losing patience with black, which shows every scuff and goes slightly grey with age, and realising navy does the same job without sulking about it.
The fit question with a bag isn't really about size, it's about where it sits and what it does to your shoulder after an hour. Try it on with a coat, not just a jumper, because a strap that's fine over bare arms can cut in once you add bulk, and anything that gapes open at the top when you're not holding the sides shut will drive you mad on a crowded bus. Cross body styles need the strap long enough to sit below your hip, not at your waist like a sash, and if there's an adjuster, check it actually holds under weight rather than slowly sliding an inch every time you walk. I bought one with a magnetic clasp that looked secure in the shop and popped open on a hill in about four minutes, scattering a set of keys I'm still not entirely sure I got all of back.
Where navy falls short is anywhere you want a bag to disappear. Black genuinely does vanish against most outfits, and navy, for all its good manners, still reads as a colour choice, so if you want the bag to be invisible scaffolding under a bold coat, this isn't it. It's also a poor match for anything with warm undertones, a camel coat or rust jumper, where it can look slightly apologetic next to the black bag you already own and reach for instead out of habit. And if you're buying one bag to do everything from the school run to a wedding, navy is a touch too casual for the second and a touch too polished for the first, it sits in the middle rather than nailing either.
Where it wins is in the wearing. Good navy leather or canvas ages the way denim does, softening at the corners and taking on a slight sheen where your hand rests on the flap, and it hides the ordinary dust and pavement grime that shows up instantly on black. Wipe it down with a damp cloth rather than anything with alcohol in it, keep it away from direct heat if it gets wet, and feed the leather every few months if that's what it's made of. Mine has gone through two winters and a dropped umbrella and looks, if anything, better for it.
I'm glad I finally gave in. It's quietly done more than I expected.