Harry Potter and the Deathly Hallows parts I and II are no doubt the darkest in the Harry Potter franchise, but the mise en scene of darkness the completly encompasses the existence of the wizarding world is executed with such beauty it is hard (for me at least) not to be completely delighted by it. It so complements the narrative and energizes the plot that it is hard not to be completely taken in by it.
The landscapes in the film are so vast and so gray they give a feeling of utter weariness to the viewer on behalf of Ron, Harry, and Hermione. The images give an amazing sense of the extent of both their physical and emotional journeys.
From the very first seen we see a lonely cottage on the edge of a cliff. All that is around as far as the eye can see is ocean and jagged rock. This look of utter abandonment emphasized the trios journey and foreshadows Harry's ultimate challenge. No longer protected by Dumbledore or the Order of the Phoenix they must pursue the horcruxes alone. Likewise, Harry must later face Voldemort without his protectors and without his friends.
The darkness feels pervaisive and endless, like it is everywhere and in everyone. Everything looks like it is cold and dark and far away. The trio ventures to Gringots, the lobby of which is oppressively beautiful, but that is not their destination. They quickly have to leave the lobby into the darkness of the vaults where their journey continues.
For example, when Lord Voldemort is walking barefoot through the room of slaughtered Death Eaters. The room is black, he wears billowing black robes, all of the Death Eaters, alive and dead, where black, but on the pale gray skin of Voledmort's feel we can catch a glimpse of the bright red blood of his followers .
When they finally make it to Hogwarts and endure the battle with Voldemort's Death Eaters, the brightness of the flames that consume the Quidditch pitch and other familiar aspects of the Hogwarts grounds are that much more haunting in contrast to the privious black and gray images.