Answer:
I would like to assume that "melancholy darkness" means something along the lines of darkness that hits out of nowhere.
Step-by-step explanation:
Okay so "melancholy" has the exact definition "a feeling of pensive sadness, typically with no obvious cause." And darkness is obviously something with no light. Something to compare this to might be depression because more commonly, the symptoms of depression can hit out of nowhere, and suddenly one is enveloped in darkness. Now, this might not be the best answer but it's how I think of it from my own personal experiences. Hopefully, this helps.