Upvoted for geek scholarship. Also, Thestral hair is used in the Elder Wand, and IIRC the Cloak of Invisibility is mentioned at least in MoR to be marked with Thestral blood. (The Stone is said to be marked, but I don’t remember if it’s mentioned with what.)

The Hallows have all sorts of attributes related to permanency, or power over decay: being “invincible” means (more-or-less) your shields aren’t broken and your attacks aren’t stopped, the wand is able to repair broken wands, the cloak hides from death (it also looks like new from the descriptions), the stone brings back “souls”, and owning all of them makes you “Master of Death”.

In the twisted logic of magic, I could see Thestral blood making the effect of a potion permanent (or at least for life) rather than time-limited like most potions seem to be. (Transfiguration needs to be maintained, Polyjuice needs to be taken periodically, and the D&D Eagle’s Splendor has defined duration.)