The room was pitch dark, so I couldn't see anything at all.
Literal
Room [topic-は] pitch-dark was [because-ので] at-all nothing was-visible.
真っ暗 uses the intensifying prefix 真っ~ to turn 暗い (dark) into 'pitch dark.' Combined with ~ので (because) and double negative reinforcement 全く何も見えなかった, this creates emphatic negation.