I think they basically a dating fest to meet eligible dubs outside your area. My brother went to them when he was single, cost about £60 dinner and dance ( or grub n a rub). He went about 10 yes ago I think most were London based often on a boat on the Thames. I think they were a fairly open secret amongst dubs
Been to a few similar get togethers here in US and can confirm that it seems primarily for expanding the dating pool but you get the occasional couple who just want to do something new. Good way to make new JW friends if you're into that sort of thing..
I also remember being suspicious when elders showed up, taking video of everyone in the room.... Hmmm... Or maybe it was just so they had something to look at when they got home, the girls were usually dressed up very nicely (and tightly, and not always entirely modestly)
I never heard of a black tie event when I was a JW but then I'm in the midlands so they were probably in London. I remember going to a D'oyly Carte performance of Gilbert and Sullivan at Sadler's Wells with a load of bethel staff and a bethel boy came in dinner jacket, frilly shirt and jeans.