Although this article is based on official information, the actual name of the article's subject is pure conjecture.
Blue Knuckles is an term used to a color palette glitch in the locked-on version of Sonic the Hedgehog 3 & Knuckles. Blue Knuckles uses Knuckles as base character, but due the coding error or corruption, the color palette changes to one used for Sonic.
Unlocking the Blue Knuckles
There are two ways to unlock Blue Knuckles in Sonic the Hedgehog 3 & Knuckles. The first choice includes simply using a Game Genie and insert the following code: HWDT-YZCO. The more complex way involves doing a glitch that comes in different variations. One includes activating the Debug Mode as Knuckles. The player then goes to Data Select and must pick up the file that contains the Chaos Emeralds being collected. Right after selecting Marble Garden Zone from the same file, the player must then press start and down right before the flashing start on the title card. On the title screen, select Level Select and pick up Sonic (01), Tails (02) or both (00) in the upper left corner's number select, and press start to enter Hidden Palace Zone (while holding down for activating Debug Mode). Right from the start, the player can activate Debug Mode and move left on the pathway and make the rotation move quickly in the direction of the deactivated Super Emeralds on the altar. At the altar, the player can deactivate Debug Mode and land on one of Super Emeralds to enter the Special Stage. When either collecting all the blue spheres to or losing, the player is transported back to the corrupted Angel Island Zone, where Debug Mode has to be activated right away and move to the bottom until the game resets automatically (if this does not happen, the player has press reset button on the Genesis).
After the automated reset, the player can go back to the Level Select menu. If the player sees the numbers "80", "81" or "82" on the Sound Test selection, the glitch is done correctly. The player can now enter any Zone as Blue Knuckles, while most of the time, the player encounters Egg Robo during boss battles and the Super State cannot be activated normally (unless the player is using Smonitors). Due the program corruption of involving both Sonic (color palette and the chosen pathway) and Knuckles (base), certain Zones cannot be completed.
In standalone Sonic the Hedgehog 3, by hacking the game or using Game Genie (ASCA-AAHC or ASCA-BA9Y), the player can play as Knuckles, but many of the playable character's content relies on the Sonic & Knuckles cartridge. The player is basically using Sonic, but there are small content featured in the game, including Blue Knuckles' panel at the Signpost.