Oh, that's an interesting thought. I see no reason that the Blessing of Light would fade with Venat's death - she didn't really renew the spell at any point in the journey so it might be permanent. But also we might not need it any more. In ...either 5.2 or 5.3 Elidibus tried to possess G'raha but can't - he says because his soul is too dense (presumably having been rejoined in the 8th Umbral Calamity his soul is denser, which suggests that he could actually awaken to the echo under the right circumstances but he hasn't yet or maybe people on the source have stronger souls even if they aren't reincarnated souls of the ancients) It's possible, having been rejoined with the fragment of your soul that was Ardbert your soul is just too aesthetically dense now to be tempered with or without the blessing of light.