I just checked my box of Alber's Yellow Corn Meal, which has been open for a year. There are a few darker pieces, maybe a couple in a teaspoonful, among the yellow specs. The darker ones are more brown than black, and they're the same size and shape as the yellow ones, so I'm confident that they're corn, not bug droppings.
BTW, I keep all my flour, sugar, cornmeal, etc., in ziplock bags or in the plastic bags in which I brought them home from the supermarket, tightly sealed with a twister. I transfer open boxes of cereal and grains (rice, etc.) to large sealable plastic containers. I haven't found a bug in anything in a long, long time, although we are occasionally plagued with silverfish in the cabinets.