I agree with this. Nightblood=sword in the cognitive realm.
As for Nightblood's appearance to those in the cognitive realm while in the physical realm, I'm less sure. Nalthis has yet to be revealed in Shadesmar. If Nightblood was Rosharn, I'd guess glowing bead. Scadrial, glowing misty sword. Nalthis? What do would that be? Glowing Tears of Edgli (aka flower pedals)?