This message from Viseguy has links to descriptions by Nathan Tardif of the difference between the labels, "eternal," "bulletproof," etc., in the Noodler's inks he has invented (and is still working on). Thanks, Viseguy!
"Bulletproof" trumps "eternal," so it seems Mr. Tardif's meaning of "eternal ink" is more or less like the "immortality" of Tolkien elves: cannot die of either old age or disease, but can be slain by non-magical poison and violence. Therefore, "bulletproof" would be an improvement on "eternal."