Devil's food cake is a tongue-in-cheek response to the airy delicacy of angel's food cake. While the other is snowy white and feather-light, devil's food cake is denser and has a rich chocolate flavor. Some versions of the cake have a distinctively ruddy color, which is unexpected in a chocolate cake. It comes from the cocoa you choose, and the choice of baking soda over baking powder.
About Devil's Food Cake
Written recipes for devil's food cake date back as far as the late 19th century, though versions of the cake were probably made before that. Most early recipes were leavened with baking soda, since baking powder didn't see widespread use until after the First World War. Typical recipes used buttermilk to activate the baking soda and were flavored with either baking chocolate or natural cocoa. Both of those ingredients are naturally acidic and would react with the baking soda to help it leaven the cake.
Alkalinity and Color
The natural hint of red color in traditional devil's food cake recipes is caused by the alkalinity of the baking soda. It's the same reaction that makes your soda-raised biscuits and quick breads brown so beautifully, but in chocolate or cocoa the naturally brown color changes to a rusty hue. This isn't obvious in recipes that use buttermilk and natural cocoa, which are both acidic, but it's visible in recipes using plain milk and alkalized or Dutch-processed cocoa. Those are typically leavened with baking powder, rather than baking soda, but call for a small amount of soda as well to produce the reddish hue.
Adding soda is a chancy way to give the cake a red tinge, because any variation in your ingredients can alter the subtle chemistry involved. Too much soda can also give the cake an unpleasantly chemical or "soapy" flavor. To avoid this, yet provide a consistently red hue, some recipes cheat. Older recipes call for shredded beets or beet juice to provide a reliable red color, while newer recipes rely on red food coloring to accomplish the same task.
When the use of red food coloring is taken to an extreme, it produces a closely related form of chocolate cake called the "red velvet" cake. Red velvet cake recipes are usually similar to devil's food cake recipes, except for the larger quantity of food coloring. Red velvet cakes are traditionally covered with cream cheese icing for an additional tang, while devil's food cake usually has a rich chocolate icing.