Death Rides a Black Horse — Lewis B. Patten

Frank Halliday was only a boy when his father died, but with his life in deadly danger, he became a man fast. Rafe Joslin, his father's longtime foreman, would inherit the vast Halliday ranch if anything happened to Frank. And Joslin seemed bent on making that happen. After several accidents, Frank hit the trail from Wyoming to Colorado, with an angry Rafe Joslin in hot pursuit, and a bloody showdoJack 2 Sis 8-13-19, Jack 2 Sis 2 Library 8-16-19n was the only way left to end things....

Alternate blurb:

Fifteen-year-old Frank Halliday's life is placed in jeopardy by a well-meant provision in his father's will which states that his long-time foreman, who has helped raise Frank, will inherit the ranch if Frank should die before he reaches the age of 21.