Absentee Actor: Black Knight is one of four games in the franchise where Dr. Eggman is completely absent (and the only one featuring the main Sonic cast including Sonic himself), the others being the three Tails spinoff games.
Franchise Killer: Black Knight's poor critical reception and commercial failure seems to have killed off this series of spinoffs after two stories.
What Could Have Been: Secret Rings began life as a Wii port of Sonic the Hedgehog (2006), with the developers experimenting on playing the game largely with motion control. Development then switched to making an all-new Sonic title exclusively for the Wii, designed to take full advantage of the hardware, and thus Secret Rings was born.
Working Title: Sonic and the Secret Rings used to be Sonic Wildfire.
For a short while between Wildfire and Secret Rings, it was also going to be called Sonic and the Secret of the Rings, but they took out the "of the" to make it a bit less wordy.