I asked the babushka who gave us those beautiful eggs how she did them. She said that she just took some random leaves, put them up against the eggs, and tied them up tightly in stockings. (I wonder if she used leaves from a flower arrangement, because I recognize fern and rose leaves?) Then she boiled them with onion skins. That's the standard way to get the reddish colored eggs that are so common for Easter in this part of the world. She didn't say that she added vinegar, but most people do. Apparently they turned out so dark because she started with brown eggs instead of white. It sounds so simple that maybe we'll try it this week.
I do have vague memories of doing something similar with Mom when I was little.