This is where I see it making sense. If you live in an area where daytime, summery weddings are the norm you are more likely to think black isn't really "appropriate" for the event since most everyone else will be in bright colors, florals, etc. But, if you live in an area where weddings are more formal, in the evening, indoors, you are likely to see a good percentage of guests in black dresses.
Why read extra meaning into things unless you know 100% for sure the intention behind it?
I could understand if this took place in a small community with very specific cultural rules that everyone followed. But, nowadays people move all over the place and even travel to attend weddings, so it's likely that the guests will be from different places with their own traditions. Based on this thread, three quarters of people have absolutely no idea that there would be any controversy or issue with wearing a black dress to a wedding. So, why would anyone's first thought be, "She must have purposely chosen her outfit to publicly display her disapproval of my wedding" instead of "Oh, she must not know that around here we don't typically wear black to a wedding"?