In my opinion, the mood of "The Speckled Band" is more malignant and dark than the mood of either of the two other stories that you mention.
In "The Speckled Band" the mood is more malignant and dark for at least a few reasons.
- In "The Speckled Band" we know from very close to the start of the story that there has been a murder. We may suspect it in "Lip" but we do not know.
- In "Band" Helen Stoner tells Holmes right off that it is "terror" and "fear" that make her shiver.
- In "Band" Helen is living on this estate where it's dangerous to go outside at night because of the wild animals. This makes it seem quite dark as well.
The mood of The Adventure of the Specked Band owes a great deal to the traditions of Gothic literature. The Gothic is a sub-genre of Romanticism: a movement that sprang up in late-eighteenth century Europe in opposition to the rationalism of the Enlightenment. Gothic literature is characterized by mystery, horror, and a suggestion of the supernatural. Its plots often involve murder, dark secrets, and young ladies in danger from powerful men whose sinister agendas are hidden. Vast, crumbling buildings (like Stoke Moran) are often central to Gothic stories: so much so that they serve almost as characters in their own right. The Gothic (like Romanticism in general) emphasizes feeling over reason, and makes use of powerful emotional responses — both positive and negative — to nature.
Conan Doyle uses all of these elements in The Adventure of the Specked Band. Helen Stoner arrives at Baker Street terrified and desperate for help, “dressed in black and heavily veiled.” She knows she is in danger, but she feels helpless to define or combat the threat. She is clearly a heroine in the Gothic style. The violent and secretive Dr. Roylott is a classic Gothic villain. Julia’s mysterious death, the isolation and derelict state of Stoke Moran, and the sinister whistling sound that the sisters hear in the dead of night all contribute to the atmosphere of the tale. As mentioned above, the emotional response to nature is a defining characteristic of the Gothic. Conan Doyle puts this at the center of his story by giving his villain a pair of frightening pets (a cheetah and a baboon), and then by using a deadly swamp adder as the instrument of murder.
The Man With the Twisted Lip contains some Romantic elements — Boone’s grotesque disguise, the sinister opium den — but these are not specifically Gothic. It focuses less on emotion and more on Sherlock Holmes’ intellect. The Adventure of the Blue Carbuncle is even less likeThe Speckled Band. Here, a lot of the reader’s pleasure comes from watching the great detective using reason to solve a puzzle. It is set in London during the Christmas season, which gives it an atmosphere more like Dickens than like Poe.