Board Thread:Suggestions forum/@comment-198.180.129.13-20171205180251/@comment-33185350-20171211160702

The Black Breath is not a poison put on arrows, but rather a terrifying malady incurred from encounter with a Nazgûl.

“...and they called it the Black Shadow, for it came from the Nazgûl.” - The Return of the King, The Houses of Healing

However, Faramir’s case of Black Breath does not come from an arrow.

“‘He is nearly spent,’ said Aragorn turning to Gandalf. ‘But this comes not from the wound. See! that is healing. Had he been smitten by some dart of the Nazgûl, as you thought, he would have died that night. This hurt was given by some Southron arrow, I would guess.’”

“‘He is a man of staunch will, for already he had come close under the Shadow before ever he rode to battle on the out-walls. Slowly the dark must have crept on him, even as he fought and strove to hold his outpost.’”

He has poison from an arrow from the Haradrim. However the healers also decide that he got the black breath overtime, not from any arrow.

Also note Éowyn and Merry get the Black Breath from striking the Witch-king, not getting hit by an arrow or any injury. For Éowyn:

“The arm that was broken [this is the one the Witch-king hit] has been tended with due skill, and it will mend in time, if she has the strength to live. It is the shield-arm that is maimed; but the chief evil comes through the sword-arm. In that there now seems no life, although it is unbroken.”

So the Black Breath doesn’t just come through wounds at all. With regards to Merry:

“...And he has taken a hurt like the Lady Éowyn, daring to smite that deadly thing.”

The Black Breath seems to be a thing from contact with the Nazgûl, not necessarily a physical poison put on weapons. It also can come from their general presence, even if distant, over a period of time. Faramir’s fever doesn’t come from the Black Breath, but a regular poison on a Haradric arrow. The idea of Morgul Arrows has been promoted by the Hobbit movies but is absolutely not correct. Crafting arrows with Black Breath would not make sense.