I know the blood is used to mark the Deathly Hallow symbol on the Cloak, but could you remind me where it says this relates to permanence?

Hmm, rereading the section of his Azkaban trip where Harry was making his Cloak related discoveries I seem to have confused the fact of the thestral blood symbol empowering the Cloak with someones conjecture in some discussion thread after we learned the law of potion conservation that the thestral blood suggested as a substitution in the eagle's splendor potion could have served as a modifier to make it permanent.