Post-production
In the DVD commentary for this episode, director James Contner says that while filming the scenes with Willow and Xander on the cliff, the wind churned up dust, which blew into Alyson Hannigan's eyes. Because of the dust, Hannigan removed her black contacts and her eyes were blackened digitally in post-production.
Read more about this topic: Grave (Buffy The Vampire Slayer)