(Harry thinks it's the Thestral blood, and guesses that Thestral blood is also responsible for the permanency of his Cloak's magic.)

Not quite.

Scrawled in the margin was a handwritten annotation saying, I wonder what would happen if you used thestral blood here instead of blueberries? and immediately beneath was a reply in different handwriting, You’d get sick for weeks and maybe die.

Some chapters later:

For Bellatrix Black and the snake draped around her shoulders were concealed by the Cloak of Invisibility, one of the three Deathly Hallows and reputed to hide its wearer from the gaze of Death himself.(...) And Harry knew, now, that the concealment of the Cloak was more than the mere transparency of Disillusionment, that the Cloak kept you hidden and not just invisible, as unseeable as were Thestrals to the unknowing. And Harry also knew that it was Thestral blood which painted the symbol of the Deathly Hallows on the inside of the Cloak, binding into the Cloak that portion of Death’s power, enabling the Cloak to confront the Dementors on their own level and block them.

These are the only two mentions of Thestral blood in the story. They don't make it sound likely that Lily went on to use Thestral blood in the potion, and Harry doesn't make that connection either. The association Harry does make seems to be between Thestrals and Death and possibly invisibility, not permanency.