Board Thread:Show-Related Questions and Answers/@comment-25926288-20151120003425/@comment-25536775-20151122042854

Well, insofar the only means other than the Sorcerer's door are Pegasi feathers, the Shadow, the Dark Curse, Magic Beans, and Mermaids. It's completely possible that Blue wasn't aware of mermaids and the Shadow, and Magic Beans and Pegasi were implied to be extinct.

It's also worth noticing that Nimue wasn't tested for the grail. For all that we know, it's not the grail itself, but the stone it is placed in, that tests one's worth. Otherwise, why would Merlin even bother to hide the grail since whoever was unworthy wouldn't manage to drink it, or why Zelena and Emma were able to just hold Excalibur like that, despire never having been tested, after Arthur and Rumpel gave it to them?