Some Dum Dums have wrappers with question marks where the flavor is normally printed. This was a marketing idea that made the production process run more smoothly and made eating Dum Dums more fun. The Mystery Flavor pop is a mixture of two flavors that come together when the end of one batch of candy meets the beginning of the next batch. Rather than shutting down to clean out the candy equipment between flavors, Spangler turned lemons into lemonade and made pops out of the combination of flavors — the tail end of the old, and the beginning of the new. The candy lines keep running continuously, and the Mystery Flavor pops are a surprise treat every time.
The more you motherfucking know! And there's more info about Dum Dums here.