We've already tackled the tricky topic of whether it's ever appropriate to wear white to a wedding, but what about that other divisive color choice, black? This is far less cut and dry and really depends on the type of wedding you're attending. Before you slip into that LBD, do think about how appropriate it might be.
If you're attending a traditional church wedding, wearing black can be considered a big faux pas. Though things have moved on a lot in the last few decades, black in church is still associated with mourning, and the older generation in particular may sneer at your choice. Proceed with caution — nobody wants to annoy Grandma! That said, maybe your friends and their family aren't hugely religious, and they're just marrying in church because that's what people do. Let the bride and groom guide you here: if you know they're quite traditional and religious and likely to sweat the small stuff, avoid black. If you're desperate to wear a dark color, try navy or charcoal instead.
If the wedding is a more modern affair, perhaps at a registry office or historic hotel, especially in an urban setting, then chances are you won't be the only one wearing black. Evening weddings, in particular, tend to have a more relaxed atmosphere and the old-fashioned rules don't apply.
For garden or beach weddings, or anything held in Summer in an outdoor setting (a marquee or a country hotel with beautiful grounds), a lighter color will probably feel a lot more appropriate and look much better in photographs. Most people tend to opt for color at weddings, so you could end up being the only person in black, adding a morose touch to those atmosphere snaps! Though most of us wear black throughout the year, warm weather gives us all an excuse to break out the pastels, florals, and lighter hues, so definitely consider other options in the warmer months. In Winter, you'll probably find you're fine.
If you're just not the kind of person to wear florals or a bright color, you have two choices. First, put the bride and groom's feelings ahead of your own and make an exception for just one day (especially if they are close family). Or, wear black, but try your best to give it a wedding-appropriate spin; choose a romantic fabric like lace or chiffon, pick a subtle floral on a black background, opt for stripes or a dark print, or coordinate your black dress, jumpsuit, or skirt with other colors: a lighter-colored jacket or cardigan, a pair of pretty heels, or delicate jewelry. Finally, you could always wear a suit: that's what all the men will do and there's nothing to say you can't join in!
The rules on black at weddings vary wildly depending on location, so this advice may not work everywhere. In the USA, it might be much more acceptable to wear a pretty LBD at a wedding, but if you're in the UK, this varies. If in doubt, ask a member of the wedding party!