Without light. Also of material that is hard to look at — which in this house is witnessed with honour rather than flagged.
Old English deorc (dark, gloomy, sinister); without light, extended early to material that is hard to look at directly.
Old English deorc already carried both senses before this house used the word at all: an absence of light, and by the same breath, a thing hard to face. This house keeps the older honesty of that pairing — the difficulty is named, never hidden behind a softer word.