You’re in the UK, though, right? And aren’t most weddings in the daytime there? Evening is much more common here, at least in my community, so the black I’m talking about is party clothes. You’d see way less black in the US at a daytime wedding, too.
If there is a party at night, still no black?
Bear in mind that if you hold a wedding in the U.K, unless it’s in a very remote location the majority of the guests can probably set off from their homes in the morning and get home the same night. That means they have no hotel room to change in. So if you’re planning to have dancing and a party going on till late you’ll probably have the wedding ceremony in the early afternoon, dinner in the early evening, with dancing to follow. Whatever the guests turned up wearing, they wear till they go home.
That's the same here. I'd say most evening weddings ceremonies we attend start somewhere between 5 and 7pm before moving on to the reception. If an afternoon ceremony, then the dress code is usually more casual.
And although I know there are exceptions in some communities, it's much more common in the US, at least where I live, there is never a gap between the ceremony and the reception. Sometimes the reception is in a different venue, but everyone immediately goes there from the ceremony. Most common would be something like a 6:30 pm ceremony followed by a dinner reception, or 12 or 1 pm ceremony followed by a lunch or brunch reception. Sometimes I see a 4:30 ceremony followed by a dinner reception; people wear evening clothes (and I always feel kind of funny in the car on the way there!). But the point is, either all daytime or all evening, not a morning or early afternoon ceremony and an evening reception.
So that may explain why black is less common in the UK; even though a black party dress is fine for an evening reception, it wouldn't be appropriate for a daytime ceremony, and a daytime black outfit is more likely to look funereal.