The mystery flavor solves a logistical problem
In manufacturing you want as little machine downtime as possible. When the machines aren’t running they aren’t making your product, and you aren’t as profitable as you could be. It’s all about efficiency. The Akron Candy Company of Bellevue, Ohio created Dum Dums lollipops in 1924, eventually selling the product to the Spangler Candy Company in 1953. There were originally seven flavors but they broadened out to 16 flavors. To maintain flavor integrity the machines must be cleaned between flavors – this removes any remnants of the previous flavor and prepares the machines for a pure new flavor. This also creates machine downtime.
The solution: the mystery flavor. To maintain machine efficiency you want to start the next flavor right as you finish the previous flavor. Instead of shutting down the machines, the mystery flavor is created in this liminal time when two flavors are moving through the same machine at the same time. The end of the one flavor and the beginning of the next mix together in different amounts creating ever-changing new flavors. The Dum Dums’ mystery flavor isn’t just one flavor. The mystery flavor is always an unpredictable mix of flavors. It’s a fun game to guess which two flavors are making your version of the mystery flavor, and it’s a clever production solution.
Added info: In 2015 Spangler ran a limited time campaign and produced three specific flavors, outside of the normal flavors, to be the mystery flavor. These were pizza, popcorn, and bacon flavored Dum Dums. Also the name Dum Dums was chosen because a sales manager felt it was a name children could both say and remember.