This was used by a member of the 16th Michigan at the Battle of Chancellorsville to describe the night. Soldiers often use animals to represent their thoughts. I had not heard this "dark as a bag of black cats" before.
"I tell you, sir, he is as crazy as a March hare." Spoken by General Ewell to a subordinate in reference to General "Stonewall" Jackson, during the 1862 Shenandoah campaign. However, hindsight tells us Jackson was actually crazy like a fox.