These are all around grade 6 (some a bit above, some a bit below):
J. S. Bach: Prelude and Fugue in Em (Book I). This is one of the most beautiful of the set and also one of the easiest (not easy at all: easiest amongst the 48 ).
Scarlatti: There are several sonatas amongst the 555 that could be described as dark and moody. Try K 69; K 87; K 213
(All of the 555 sonatas are excellent technique acquisition exercises, and all of them are excellent repertory. See this thread for suggestions:
https://www.pianoforum.net/cgi-bin/yabb/YaBB.cgi?board=repo;action=display;num=1077145772)
Mozart: Fantasia in C minor (K475), arguably the darkest and moodiest of all Mozart’s compositions.
Mendelssohn: Songs Without Words (try Op. 30 no. 1; op. 30 no. 6; op. 53 no. 3; op. 62 no. 3 and op. 85 no. 4)
Schumann: Album for the young Op. 68 has some real gems (try no. 28 and no. 30), also check his Bunte Blatter op. 99 (try no. 2, no. 4, no. 9 and no. 10) and the Albumblatter op. 124 (try no. 8 and no. 14).
Chopin: Preludes Op. 28 no. 2, no. 4 and no. 20 are suitably dark. The easiest nocturne (grade 6) is no. 21 in C minor.
Prokofiev – Have a look here for a lot of easy Prokofiev (reply # 15). Some of them (especially the “Visions fugitives” are dark and moody)
https://www.pianoforum.net/cgi-bin/yabb/YaBB.cgi?board=stud;action=display;num=1077907916Sibelius – His ten pieces for piano op. 76 are within your capacity.
Preisner: 10 easy pieces for piano. There are some really beautiful, evocative pieces in the set. I am not sure how dark they are, but they are certainly moody, haunting pieces. (I particularly like no. 1 “A good morning melody”, no. 2 “Meditation”, no. 4 “Talking to myself”, no. 6 “About passing” and no. 10 “ A good night melody”)
Satie: Gymnopedies and also the Gnossienes.
This should get you started.
Best wishes,
Bernhard.