Please! Please make the "weapon knife" go away! True-Love makes the comment (as a linguist) that "weapon knife" is a highly improbable construction. (sentencewise that is). He did hear the theory about it back in the 60s, but makes the point that it wasn't suggested by a linguist. You just don't find examples of the words used that way. (Neither in ballads - as Malcolm points out, nor in other written sources).

Thank you every one for all the input.
Clint - True-Love did mention the blueing of guns. We wondered if that was the connection.
Such an interesting thread this has turned out to be. So glad I came here. Cheers, Joy