Ask whether the shade of the evening could come from anywhere but Qarth.
Ask the archmaester for any books he has about warlocks - you won't go home ignorant.
"It is sacred. I've never heard of them using it outside the House of the Undying," the archmaester says. "Does your [lord/lady] have enemies in Qarth?"
Ask what shade of the evening is.
Thank the archmaester for his help and hurry home.
Shake your head in bafflement - your [lord/lady] has never been near Qarth.
The archmaester scowls. "Turning blue and naming yourself 'warlock' doesn't make you magical. All things have an explanation. Remember that."
Wonder whether shade of the evening is just a drug or if it has magical powers.
"A drug used by the warlocks of Qarth," the archmaester explains. "They believe it opens their eyes to the past and future and gives them powers."
Days later, after many unsuccessful tests, the archmaester wakes Jon at dawn. "Your [lord/lady] wasn't poisoned. [He/She] was drugged. Shade of the evening."