The Lesser Rings

[jallanite]

From The War of the Jewels (HOME 10), page 383:

In S. the word gûl (equivalent to Q ñóle) had less laudatory associations, being used mostly of secret knowledge especially such as possessed by artificers who made wonderful things; and the word became further darkened by its frequent use in the compound morgul ‘black arts’, applied to the delusory or perilous arts and knowledge derived from Morgoth.

This explanation suggests, but does not prove, that when Gandalf uses the term “Morgul-knife”, he is referring to the knife being a knife of black magic rather than to it being made in Minas Morgul. Of course Gandalf might have meant both at once.