adv

• (figuratively) In a manner which is not readily visible or noticeable; inconspicuously.
  "Set defenses permitted a coach to plan an attack with scant worry as to where he would find the offensive men. So our Tom Reilly schemed darkly and came up with the attack which pried apart the Brown Bowlers. His scheme was to double team every man in the defensive line by employing one back as an eighth lineman and spreading the enemy further by posting a wingback outside the strong end of an unbalanced line which had five men on this side of center."
• (figuratively) In a manner which retards or prevents discernment; clandestinely.
  "His secret wooing of the Norman lady who was so proud and stately was very dear to him, and the romance of it pleased him even more than the mere material joys he hoped some day to gain from it. Proud as she was, womanlike she at least deigned to listen to him, and his crafty brain schemed darkly to take opportunity as it came, and make her his own by treachery."
• (figuratively) In a manner which is difficult to understand, or which retards or prevents understanding; incomprehensibly.
  "Secondly, The Covenant was then revealed more darkly and obſcurely, but now the diſpenſation of it, is more cleare and evident: the light is now marvellous, it is as the Sunne ſhining at noone-day. […] Though it was revealed before, yet it was but darkly, but now it is revealed more clearly ſince the coming of our Saviour Chriſt; […]"
• (figuratively) In a manner which tends to produce uncertainty or confusion; bewilderingly.
  "Here, as Genevieve sat musing, in apparent slumber, on her situation of peril, and her hope of liberty, her sentinels, who had hitherto walked at a surly distance from her, now chanced to pass nearer; and she heard one of them say, “She sleeps, thou need'st not whisper; -I tell thee it must be done to-night.” “What must be done?” said his companion, “for hitherto thou hast spoken thy purpose darkly.”—“The queen must be removed this night,” said the other. “Here have been missives from the bishop, who, amid the thunders of the siege, found leisure to receive and answer the letters of King Philip, touching the matter of the queen's thraldom, and to send the order thou wottest of, and which must be suddenly obeyed.”"

adj

• (chiefly poetic) Relating to darkness or obscurity.
  "A Scanner Darkly"