User blog comment:Disfigure/This was only shown in the Anime about Guren Ichinose/@comment-84.73.60.64-20160131005307

Maybe it has to do with the holder of the weapon?

As you stated, vampire's swords change their colour to red after drinking their blood. In Guren's LN 6 Krul turns Mahiru into a vampire, making her like Mika (since Krul turned him into a vampire aswell). So if Guren is possessed by Mahiru-no-Yo and we can say that the sword drinks her blood, it would make sense that the sword would turn red like a vampire's.