Found something interesting in a "home ec" radio program script from 1932 that shows how times have changed. The subject is foods for the Fourth of July:
"When it comes to food, red and white are the colors to count on. Blue is too difficult generally to work in the menu. Yes, there is a perfectly safe blue vegetable coloring on the market that is often used for candies and so on. But in general food colored blue looks too unnatural to be appetizing. I remember attending a party once where the ambitious hostess had made a red, white and blue layer cake. Bottom layer tinted red, center layer white, top layer colored blue. And there was blue icing. But I can’t say the result was successful. That blue looked far from tempting. In fact, the whole cake was a rather ghastly sight."
I'm surprised she doesn't recommend blueberries, which are used today in "patriotic" desserts. (Maybe they weren't readily available across the country then---?)