Magazine  (Sep 1977)  Page(s) 21.  
 
"Botanical Descriptions of 40 Saunders Lutea Hybrids" by Dr. David Reath and Silvia Saunders
...These descriptions aim to provide the nurseryman and the average gardener with a means of determining the differences between the hybrids, many of which are very similar indeed...
Heart of Darkness—(One of the very rare F-2 from Dr. Saunder's days). Very single, somber dark maroon, with dark center. Heaviest petal substance of them all.

Article (magazine)  (Jan 1955)  Page(s) 23.  
 
All the present hybrids are sterile or at least very nearly so. They often produce seed pods without any seeds inside, or if there are seeds, they are hollow. There are occasionally a few apparently good seeds, but they do not germinate. At least that is the story so far, with only a small number of supposed exceptions. Four of these supposed exceptions were raised by Professor Saunders. One of these, 'Heart of Darkness,' is a flower worth growing for its depth of color and richness of petal quality.

Article (magazine)  (Jan 1955)  Page(s) 25.  
 
Varieties Raised in America. All by Professor Saunders...
The "Black Pirate Group." Crimson to very dark, almost black maroon, single to double...
'Heart of Darkness,' (1948), second generation hybrid.

Article (magazine)  (Jan 1955)  Page(s) 57.  
 
Alphabetical Check List of Tree Peony Names In  Public Collections And/or Available in Nurseries 1954-1955
Lutea hybrid, Crimson, 'Heart of Darkness' (Saund. 1948)