The blade was once blue. After he fell to the dark side, Ben attempted to make his lightsaber’s kyber crystal bleed. This process is what gives the lightsabers of every Sith Lord their red blades.
Osha Turns to the Dark Side in "Star Wars: The Acolyte" Star Wars ... Force-choking Sol to death while bleeding the kyber crystal in his lightsaber from blue to red. Osha’s dark side turn ...