This powder was either nonexistent, or when looked at, a harmless substance.
But when I was pondering a woman denounced to the Inquisition as a witch in Renaissance Venice -- and tried and condemned by them as a poisoner. (She had discovered that something they used to remove fleas was also poisonous to humans and given it to her clients, resulting in at least three deaths.)
And while the line between poison and magic was a bit loosey-goosey -- poison, after all, worked by unknown means -- a magical powder would probably appear harmless. After all, you didn't want to put it into its final, malignant form until you decided exactly what you wanted to do. It might even vanish when searched for by your enemies.