I’d give the novel a little space/time, probably three weeks, or enough time for me to go write a short story. Then I’d come back and reread it. Does the darkness feel right/needed? Have I been threading it in all along, or was it really random? If it’s the first I’d have to make a very tough decision to pull out not just the dark scene, but all of the foreshadowing to that scene. Or decide that the foreshadowing was there for a purpose and that this is the way the novel will be going. If I make the second choice I will also decide if I want to keep working on the novel at this time, or if I want to put it to the side and work on something more light-hearted.