Sometimes packages get in under the radar, depending on what exactly is on the customs form, but yeah, you're probably going to get billed duty. What bugs me more are the handling fees from couriers when this happens.
My gf has something similar in "midnight blue". Which is a posh word for dark-as-hell blue.
It's a useful colour though; historically, the thinking behind using it for evening items is that it doesn't get that greeny-yellow cast to it under artificial lights that cheap black dyes can sometimes get. Midnight blue instead just looks like a true/jet black under those conditions. Modern artificial lights and better black dyes make this factor less relevant these days, but I still quite like it as a shade. I have a tux in midnight blue that definitely exhibits this behaviour of appearing black at night. I also have a regular suit in midnight blue but I use that more during the day for work - where it appears like the darkest possible navy - than I do for evening events.