Update - After I tried on all dresses and looked again at some of the online stores I'd looked at before (Boden, Kate Spade, J. Laughlin, etc.), I still prefer the Kay Unger black dress. When I spoke to D, she said she wouldn't necessarily wear a black dress to a summer wedding, but I could do whatever I want. Interestingly, when I looked at some dress sites, when I filtered in MOB dresses, there were plenty of black dresses, so I don't feel like it would be a faux pas.
That all being said, my birthday was Friday, and D got me a gift certificate to get my color wheel done (she did it herself a couple of months ago and really enjoyed learning about it). So I uploaded six photographs that best represented my skin color, eye color and hair color (they do ask you what your hair color was as a child), and I got my color wheel last night. They suggest that black is too cool of a color for me, so the black dress might be out. So it also suggested some colors that I wouldn't have thought look good on me (I've never tried any of these colors on). With this in mind, I have reached out to a seamstress to see if she can make the black dress, but in a fabric that is more suited to me. So I'm pursuing the option of having it made for me. With 2.5 months to go, I should be fine. It's really too bad it's not available in the teal in my size because the teal is in my color wheel.
The only other hitch is, when D found the Kay Unger dress for me, she also found it in white, and ordered it for herself as a possibility for the 'rehearsal dinner' (read... night before the wedding dinner, there won't be a true rehearsal). She likes it for herself, but has ordered at least a half dozen other dresses and says if she likes another one, she would forego the Kay Unger so I could wear it.
My second vaccine is scheduled for this Friday, so two weeks from then, I should feel safe spending some time in a retail store if working with a seamstress doesn't look like a viable option.