The name is actually sort of a double entendre.
While it is made with stewed prunes, the reference is also intended remind us of Prunella vulgaris, or self-heal, an edible plant used in salads, as a spice, or in tea-like infusions. The icing would have a similar purple color.
An early recipe for the cake appeared in the March 4, 1932 edition of the El Paso Herald-Post:
Blend and cream one-half cup of shortening, one cup of sugar and add two eggs and two-thirds cup of chopped stewed prunes. Stir in two-thirds of sour milk. Add one nad one-third cups of flour, one-half teaspoon each of soda, salt, cinnamon, nutmeg, allspice and baking powder all sifted together. Pour into two layer pans. Bake in moderate oven (350 degrees F.) about 25 minutes. Cool. For creamy icing, mix two cups confectioner’s sugar, one-half teaspoon of cinnamon, one-eighth teaspoon of salt. Combine half with two tablespoons shortening. Add remaining sugar, two tablespoons of prune juice and one tablespoon lemon juice. Beat until creamy. Spread between layers and on top and sides.
Mrs. Jessie Orrison.
From a box sold in Winsted, Connecticut.
1/2 cup Crisco
1 cup sugar
2/3 cup sour milk
1-1/3 cup flour
2/3 cup stewed prunes
1/2 teaspoon [each] baking soda, salt, nutmeg, allspice, baking powder
Cream Crisco, sugar, and eggs. Add chopped prunes. Add milk. Add sifted [flour?].
Pour into 2 greased large pans. Moderate oven 25 minutes; cool.
2 cup confectioners sugar
1/2 teaspoon cinnamon
1/8 teaspoon salt
Combine half with 2 Tablespoons Crisco. Add rest of sugar, 2 Tablespoons prune juice, 1 Tablespoon lemon juice. Beat until creamy.