It's taken a little while, but I finally found a pair of black flats that will look OK with dress slacks. I liked a different pair a little better, but the only color they had left was brown.
After I brought them home I thought they might be a little young for me, but what the heck! They are black, they are cute, they are comfortable and they are flat. What more could I ask for! (A 3-inch heel would be great, but that might defeat the purpose. Repeat after me: flat shoes = healthy feet, flat shoes = healthy feet... )