Should Anakin’s lightsaber be red during the duel on Mustafar?
If this is coming from the new canon explanation for red lightsabers (“bleeding” activated blue/green/purple/whatever crystals), that’s not something that just “happens” when you’re using the Dark Side. It’s a specific ritual that a Sith does to bind someone else’s crystal to them. Anakin hasn’t done this ritual to his lightsaber’s crystal (and probably wouldn’t even need to, since the crystal is already bound to him), so it would stay blue unless he took the time to sit and force the crystal to bleed.
Also, this is (I think) the official explanation for Kylo’s unstable blade - he did a shitty job of “bleeding” the crystal, and it cracked, which also meant he had to switch to an older design of hilt with side vents (the “crossguard”).