From Sonic Retro
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He plays like regular Knuckles, though he uses Sonic's palette and Sonic's life counter in the HUD. Although he can complete several levels just fine, many glitches usually arise, especially when it comes to levels in which there are Sonic-specific or Knuckles-specific paths, as it's possible to get stuck and/or to be unable to trigger certain level events. To add even more confusion to the mix, he is referred to as Tails in the "(character) Got Through" text at the end of each act (as pictured). The result screen is often glitched as well, with random garbage tiles appearing all over the screen.
Super Blue Knuckles flashes yellow just like Super Sonic; interestingly enough he reverts to red once the super transformation is gone, and he'll stay red up to the next zone, unless he dies. Hyper Blue Knuckles flashes all the colors of the Chaos Emeralds just like Hyper Sonic; he has (glitched) afterimages and he can breathe underwater like the other hyper forms in the game (Hyper Sonic and Hyper Knuckles). Once again, he reverts to red once the hyper transformation ends. However, obtaining Hyper Blue Knuckles by placing and breaking an S monitor via debug will result in an intermediate form between Super and Hyper, as he will have the glitched afterimages, but he will just flash yellow and he won't be able to breathe underwater.
Finally, Blue Knuckles makes an appearance on the signpost at the end of act 1s in Sonic 3.