Interesting commission, Gail. Isn't black the color used for marriages in some cultures - they're just as aghast that we use white for ours (which I think is a color of mourning in China).
Richard's india ink idea is a good one. I was going to ask if they have the funds to spring for ebony veneer.
I'm a consultant by trade, so I tend to be on a lot of sales calls. You may be able to steer them. The first wood that popped into my head when you described what the wanted was bloodwood. Blood - Heart... Maybe something with a nice understated brass inscription - maybe a couple lines from their vows.
Maybe you could do a box in a box. Black and white to match their kitchen, with the 2nd box inside it.
And I totally agree with Art's two rules. I have to live them every day.
Where did I put that tape measure...