What the word "black" refers to in the case?
I'd like to ask about the sentence below from A Case of Identity by Conan Doyle.
while the lady herself loomed behind his small black figure like a full-sailed merchant-man behind a tiny pilot boat.
What I want to know is, what the word "black" refers to in the case. In this sentence, "his" refers to a hired servant boy, working in the building where Holmes lives. So he had a small body, and wore black uniform, or he was black skin color? The same boy was referred to as "a boy in buttons" but I'm not sure this "in buttons" means too. Is this some kind of uniform whose color almost always is black or some other dark hue? There's no more reference to this boy at least in this episode. So can anyone be sure about the author refers to the boy's skin color here or the color of what he wears? Thanks.
It refers to "figure", meaning the outline of a person (with clothing and whatever else they're wearing).
See definition 2b here of "figure".
Based only on this description, we can't be sure of the boy's skin color, or the color of his clothing. As pointed out in another answer, it says elsewhere in the book that the boy was rescued from a workhouse. "Boy in buttons" indicates that he's wearing his uniform from his former school. Some possibilities (not mutually exclusive):
Possibility #1: The servant boy was back-lit (silhouette). But given that the lady behind him was not described in the same way, it's not likely that the boy was back-lit and the lady was front-lit.
Possibility #2: This was a comment on the overall darkness of the servant boy's appearance relative to the lady, as viewed by the narrator. For example, if he was wearing dark clothing and his face was dirty, his figure might be described as "black". Maybe the lady behind him was wearing bright clothes, so the boy's appearance starkly contrasted with the lady.
Details about the servant boy's appearance might be deliberately vague. His appearance is likely only useful as a contrast to the lady: to illustrate that the lady has a certain large and boisterous presence, or to illustrate that he was announcing a person while that person was, behind him, very obviously visible to Holmes.
Somewhere else in the story I noticed that the child said he was rescued from the workhouse (equal opportunity misery). Unless it's important to the story elsewhere, I can see this referring only to the child's all-black uniform
https://www.shmoop.com/sherlock-holmes/appearances-quotes-2.html
As he spoke there was a tap at the door, and the boy in buttons [emphasis mine] entered to announce Miss Mary Sutherland, while the lady herself loomed behind his small black figure like a full-sailed merchantman behind a tiny pilot boat....
Look at all the subtle ways in which Mary Sutherland's slightly ridiculous character is suggested by her appearance: she "loom[s]" like a "full-sailed merchantman" (a kind of ship). In just a few words, Sutherland appears (a) big, and (b) funny. Holmes may be a great champion of unbiased observation, but Watson, as our narrator, gives his opinions about people's looks all the time.
https://www.thefreedictionary.com/merchantman
mer·chant·man (mûr′chənt-mən) n.
- A ship used in commerce.
- Archaic A merchant.
mer•chant•man (ˈmɜr tʃənt mən)
n., pl. -men. a trading ship.
https://books.google.com/books?id=0eM1AQAAIAAJ&pg=PA169&lpg=PA169&dq=%22boy+in+buttons%22,+meaning&source=bl&ots=7Ii8Rc3CB3&sig=ACfU3U3jqWeXntmcGL4Kg4JWZBQ3cnIzjQ&hl=en&sa=X&ved=2ahUKEwjTz9Wf4orlAhUj01kKHfxjAwE4ChDoATABegQICBAB#v=onepage&q=%22boy%20in%20buttons%22%2C%20meaning&f=false
A Dictionary of Phrase and Fable Ebenezer Cobham Brewer (1923)
A boy in buttons. A page, whose jacket in front is remarkable for a display of small round buttons, as close as they can be inserted, from chin to waist.
