Muggles' Guide to Harry Potter/Magic/Peruvian Instant Darkness Powder

Overview
Peruvian Instant Darkness Powder is, as the name suggests, a powder which renders an area instantly dark when it is thrown.

Extended Description
A product of Weasleys' Wizard Wheezes, Peruvian Instant Darkness Powder is one of the more serious products that the Twins sell to the Ministry of Magic and similar organizations. We see it in the back room of the Twins' shop when Fred and George are showing Harry around.

Somehow, Draco Malfoy manages to purchase some of this material, and uses it to mask the invasion of Hogwarts from the Room of Requirement in Harry Potter and the Half-Blood Prince. When the powder is thrown at Ron, Ginny, and Neville, they are unable to see anything, which means that it must have blocked all light from entering the region where the powder was thrown; however, Draco, who has a Hand of Glory, is still able to see to lead the invading Death Eaters past them.

Analysis
On its own, this would seem to be not particularly useful; what is the use of having an area suddenly become totally dark? Particularly if you must travel through that area yourself, it seems counter-intuitive to eliminate your own ability to see what you're doing. In the book, however, Draco pairs this with the Hand of Glory, which will illuminate the resulting darkness only for him. He is then able to lead a train of Death Eaters out of the darkened area, while those set to watch him are left in the dark. It is uncertain why the darkness powder does not affect the light of the Hand Of Glory; perhaps the Hand of Glory somehow makes use of a part of the spectrum other than that affected by the darkness powder.

The mature reader can't help wondering if the name of this substance is derived from "Bolivian marching powder", a slang term for cocaine.