The first thing I check on a blue jacket is whether the blue looks the same indoors as it did near the window of the shop. So many of them turn traffic cone navy under office lighting, or go a bit grey and sad the second the sun's not on them directly. Hold it up to a window if you can, and then look at it under the worst, flattest light in the building, because that's the light you'll actually be wearing it in most days.
Where blue jackets let you down is anything that needs to look sharp and serious in one go, a leaving do with speeches, a meeting where you're pitching rather than attending. Blue, even a dark one, reads a notch more casual than black or camel, and on a bright, saturated shade it can tip into looking like you've borrowed something from a weekend coat. It also shows up badly against anything patterned or busy underneath, the two fight for attention and neither wins. If you need one jacket to do funerals, interviews and client meetings, blue isn't that jacket, and I'd rather tell you that now than have you disappointed on the day.
Where it earns its place is everywhere else. Spring, when you want colour back in your life after months of black coats, a mid blue does that without trying too hard. Early autumn, over a jumper, on the walk to somewhere that matters but doesn't need a full suit. Weekend errands, school pickup, a Sunday lunch with people you like, a Tuesday when you just want to feel less grey than the weather. It copes fine with drizzle if the fabric's got any weight to it, less fine in proper rain, that's when you notice if the dye's any good, because cheap blue goes patchy and damp looking faster than any other colour I own.
On shade, go for navy if you want one jacket that behaves like a neutral, it sits under most things without an argument. A mid denim blue is the easiest to actually wear often, forgiving with jeans, trousers, most tops you own. Steer away from anything bright or electric unless you already know you reach for colour, because it's a jacket that gets worn twice and then sits there reminding you.
The mistake is buying it in a shade you love on the hanger but never actually wear with anything else you own. Check what's already hanging either side of it before you commit, not after.