attire

I would say something along the lines of "please indulge us by wearing black." Maybe throw in an "if possible" also.

I would state something to the effect of “the couple formally requests guests wear black attire”. Do keep in mind though, there may be people who do not indulge this request.

You can simply state, "This is an all black affair" on your invitation. But as Chrysta noted, some people might not adhere to your dress code request. You might even want to go with an all black or heavily black invitation to reinforce the message.

You can ask for anything you want, but you should understand that some guests may not understand the request, some simply may not want to, and some may not be able to afford to buy something new to wear to your wedding.

Before proceeding, ask yourself these questions: Are you OK with people declining the invitation because they can't meet your restrictions? Are you planning to not allow people in if they don't comply? If the answers are yes to both, then go ahead and make it very clear to your invitees that you care more about aesthetics than their attendance. It's only fair to be up front about this so they can plan accordingly.

View Quoted Comment I like the paraphrasing of this request

I think you can state on your invite that we kindly ask that guests wear black attire, but just know that some guests may choose to wear something else. I also think it depends on how strict you are going to be as to whether this is truly acceptable. For example, are you going to be furious if a guy shows up in a black suit, but his shirt is white and his tie is green or a woman shows up in a black dress, but her heels are red and her purse is silver? I know a previous poster said she wasn't going to allow her guests to attend if they didn't follow her dress code which to me is insane so as long as you understand that they might not listen and are willing to accept that then go for it.

You can state that it’s an all black evening affair. I just recently went to an all black dress code wedding and there were some people who did nor adhere to that request. So I would just be mindful that some people may still wear what they want.

If this backstory os mostly to prevent a few people from dressing badly, you would do better to address those 20 or so people than all those you are sending invitations.

I think Chrysta nailed it on the verbiage! Keep in mind that unless it is explicit some guests will read it as “black-tie” dress code not the color black.
I’ve been to an all white rehearsal dinner party and it caused a lot of questions directed to the bride (are khaki pants ok?) so you may want to consider pictures on the website in the FAQ as examples. Also note that many women own black dresses, but a black shirt is not something every guy owns and can be truly terrible looking if it is slightly faded and doesn’t match the pants.

View Quoted Comment This has always been considered quite rule, for a wedding, to care more about people looking a certain way for your video and pictures, than care if they are there when you get married.

Correction: quite rude.