The first thing I check on a blue jacket is whether the blue holds its nerve indoors. So many look like a proper, confident navy or cobalt under shop lighting and then turn a washed out grey-blue the second you step outside, which is a particular kind of disappointing when you've already cut the tags off. Hold it up to a window if you can, or at least near a doorway, before you decide it's the one.
It's not the right choice for every situation, and I'll say the thing nobody wants to hear about colour: a strong blue draws the eye in a way black or camel never will, so if you want to disappear into a background at a work thing or a school run, this isn't it. Bright blues can also be unforgiving against very warm artificial light, going slightly muddy under the kind of orangey bulbs you get in older pubs and some offices. And if you run warm, a heavier blue wool jacket in a stuffy, overheated room is miserable, there's no fixing that with styling, only with taking it off and carrying it, which defeats the point of wearing a jacket at all.
Where blue really earns its spot is anywhere between autumn and spring, on days that are bright but not warm, that clean, cold light does wonders for it. A mid blue looks brilliant against a grey sky in a way that black just absorbs and does nothing with. It's also quietly good for the in between temperature days, too warm for a coat, too sharp for nothing, when you want something with a bit of structure over a jumper. I find it less convincing in high summer, unless it's a very lightweight cotton in a paler wash, because anything with weight to it in strong blue just looks like you're trying too hard against bare arms and sandals.
On shade, the easiest blues to live with sit at either end, a deep ink navy that behaves almost like a neutral and goes with everything you already own, or a soft faded denim blue that's forgiving and doesn't demand much thought in the morning. The ones that cause trouble are the bright, saturated mid blues, gorgeous on their own but fussy about what you put them with, and they show up every mark and crease far more than a darker tone would.
The mistake I'd steer you away from is buying it under shop lighting without checking the colour outdoors first, because that's how you end up with a jacket that looked right in the changing room and wrong on every single pavement since.