According to Wikipedia:
Because of differing recipes and changing ingredient availability over the course of the twentieth century, it is difficult to precisely qualify what distinguishes Devil's food from the more standard chocolate cake. The traditional Devil's food cake is made with shredded beets much the way a carrot cake is made with carrots. The beets add moisture and sweetness to the cake, helping it to be very rich. The red of the beets slightly colors the cake red and due to the richness of the cake it became known as the Devil's food.
O.k. That's a beet cake or a 'natural' red velvet cake, and I make a good one, but it's not a Devil's Food Cake in my opinion.
Devil's food cake is generally more moist and airy than other chocolate cakes, and often uses cocoa as opposed to chocolate for the flavor as well as coffee. The lack of melted chocolate and the addition of coffee is typically what distinguishes a Devil's food cake from a chocolate cake, though some recipes call for all, resulting in an even richer chocolate flavor. The use of hot, or boiling water as the cake's main liquid, rather than milk, is also a common difference.
Devil's food cake is sometimes distinguished from other chocolate cakes by the use of additional baking soda (sodium bicarbonate) which raises the pH level and makes the cake a deeper and darker mahogany color. Devil's food cake incorporates butter (or a substitute), egg whites, flour (while some chocolate cakes are flourless) and less egg than other chocolate cakes. Devil's food cake was introduced in the United States in the early 20th century with the recipe in print as early as 1905.
A similar cake, the red velvet cake, is closely linked to a Devil's food cake, and in some turn of the century cookbooks the two names may have been interchangeable. Most red velvet cakes today use red food coloring, but even without it, the reaction of acidic vinegar and buttermilk tends to better reveal the red anthocyanin in the cocoa. When used in cakes, acid causes reddening of cocoa powder when baked, and before more alkaline "Dutch Processed" cocoa was widely available, the red color would have been more pronounced. This natural tinting may have been the source for the name "Red Velvet" as well as "Devil's Food" and a long list of similar names for chocolate cakes.
I'm partial to Devil's Food Cake.
Here are several mid-century recipes. Sorry about the light print on the first cookbook.
I've posted many Devil's Food Cake recipes in the past, but today I have four mid-century recipes. The first recipe is for Cocoa Devil's Food Cake from How To Get the Most Out of Your Sunbeam Mixmaster (1950). I posted a "Mix-Easy" Devil's Food Cake for Mother's Day, and you might want to look at that one, too. It's pretty much the same as the following recipe. This is a good page for this post since there's a Chocolate Cake recipe next to the Devil's Food Cake recipe.
This same cookbook has a recipe for Black Devil's Food Cake, so now we have Cocoa Devil's Food Cake, Black Devil's Food Cake, and below a Red Devil's Food Cake. As you see, the following Black Devil's Food cake is made with cocoa and with the addition of strong hot coffee or boiling water.
The Red Devil's Food Cake is a variation on the Chocolate Fudge Cake on the same page, and to save space, they didn't reprint the entire recipe! It's a very small pamphlet. The baking soda is increased, but otherwise it's the same cake. This recipe is from the Recipes for your Hamilton Beach Mixer-17 Delicious New Cakes (1947). Don't you just love that someone wrote good next to the recipe? It's the same recipe I posted (but from a different pamphlet) on Devil's Food Cake Day for Mother's Day.
And one more Red Devil's Food Cake from the same mid-century period. This one is from Kate Smith Chooses her 55 Favorite Ann Pillsbury CAKE RECIPES.
Enough Devil's Food Cake recipes? Never! Have a look at Martha Washington's Devil's Food Cake which is from Capitol Hill Cooks: Recipes from the White House by Linda Bauer. This is a great Buttermilk Devil's Food Cake!
So what's the difference between Devil's Food Cake and Chocolate Cake? You decide.