McCarthy's Role in the Story:
McCarthy is a witness to the events surrounding the murder of Charles McCarthy, the father of the accused, James McCarthy. He is a suspicious character who initially gives misleading information to Sherlock Holmes. He is later revealed to have been involved in the murder himself, acting as an accomplice to the real killer, John Turner.
Turner's Role in the Story:
John Turner is the true murderer of Charles McCarthy. He had a long-standing grudge against Charles, who had fired him from his position as a manager. Turner is ultimately found guilty of the murder and sent to prison.
Blackmail is not a factor in the case.
The conflict in "The Boscombe Valley Mystery" revolves around the investigation into the murder and the revelation of the true culprit, not around blackmail.