[The look Sherlock gives John is so very clearly pitying, wondering how the man functions with such a lack of intelligence. But, for once, he doesn't vocalise that thought. It's enough that he gets to explain.]

Think, John. Think.

[His smile grows, eyes bright.]

There are multiple people who have come for a party, with pseudonyms, suggesting the party itself is illicit in some way. Either of a sexual nature, where the party goers are otherwise entangled and don't wish to be identified, or of a criminal aspect where names are dangerous. The host, Mr. Black, leaves the main room - one can assume the ballroom - briefly.

He trips, he falls, he bangs his head against a hard surface as he does so and, unfortunately, cracks his skull. In falling, he knocks aside a candlestick, which rolls in his blood, making it appear to be the murder weapon when it is, in fact, just a red herring. The guests could attest that this was an accident, but all fear to be found at the crime scene, and so flee.

This leaves our blundering police force to assume the worst. A suspicious death, with many suspects.