Lightsabers don't automatically turn red when one turns to the dark side. Anakin never bled his kyber crystal in the prequels, explaining why his saber stayed blue. Anakin's lightsaber needed to ...

Interestingly, however, Anakin's own Sith lightsaber was never shown in the prequel trilogy. Obtaining a red lightsaber does not happen automatically; a kyber crystal must be 'bled' for the blade ...