"Blue Plate Special" usually refers to a special that appears on the same day every week such as Monday-Meatloaf, Tuesday-Fried Chicken, etc. It is usually associated with comfort food and even more specifically with Diner food.
"And just how did that term 'Blue Plate Special' come about? During the Depression, a manufacturer started making plates with separate sections for each part of a meal - like a frozen dinner tray - it seems that for whatever reason they were only available in the color blue. Because they were inexpensive and saved on dighwashing, diners began using them for their low priced daily specials."
"Blue plate - special On special occasions, a meal served on blue willow pattern china became known as a blue plate special"