Wedding guest black dress

[AuntieSoap]

My niece is getting married next month - country house wedding, fairly big do.
She's specified that all guests are to wear black, although will allow a little bit of white if unavoidable.
She's had some comments about Halloween/funeral etc (not from me) but she's standing firm. Everything I look for in black seems to be wintery or work-y or not formal enough for a wedding.
Is it weird to specify a dress code like this? I haven't been to a wedding in years and wondering if it's a thing.
Any ideas for a black summer wedding outfit? I'm size 16, bit wobbly and 5'6" so nothing clingy.

Thanks for all the replies. I am going to go, she's a close relative and although I wouldn't normally dream of wearing black to a summer wedding, and I'm a little miffed about being told what to wear, there are some good suggestions here which I wouldn't have otherwise seen.

I would LOVE this theme. I’m always sad that I shouldn’t wear black to a wedding!

Yes, she's 30 and I'm about 50

I’d easily find something to wear for this as I love black dresses but I do think it’s weird and controlling and all about her image. What sort of person. dictates to people like this and thinks the “look” of her wedding is more important than peoples own free will about their clothes.
agree with pp that it won’t look as stylish as she imagined. It could be very offices or funeraly. It’s one thing Rochelle hulmes and her glamorous mates dressing in a theme (see her Insta second wedding or whatever it was) but it’s quite a different thing having your old family and friends of all ages, shapes, sizes, colouring, tastes, being told to wear the same thing and expecting it to be instagram perfect and stylish.
one of the things I like about weddings is that people get the chance to dress up how they like.

In case anyone else in your party is still looking, Skall Studio has a nice selection of definitely non-Wintery, relaxed black dresses. The plainest would be an excellent foil for grand jewels, but others are all lace and ruffles.

Went to a black dress code wedding 15 years ago.
The photos looked amazing and everyone embraced the look.

https://www.houseoffraser.co.uk/brand/gina-bacconi/lanie-tiered-chiffon-dress-aa3257#colcode=AA325700

It's all about the insta. Better start practicing the trout pout OP!