This is an excellent movie.
Blue (Terrence Stamp) is an American rescued (actually kidnapped as a child) by the leader, Ortega (Ricardo Montalban), of a Mexican band of criminals. He is separated from the gang in a failed raid, meets the beautiful and lovely Joanne Morton (Joanna Pettet), and falls in love with her. His former gang and his stepfather want him back, and at any cost to him and anyone who stands in the way or by his side.
The movie Blue is based in an excellent screenplay by Ronald M. Cohen, that provides a good story and interesting dialogue for the film's main actors, many of whom were very familiar because of their extensive TV exposure at the time. Karl Malden, Joanna Pettet, and Ricardo Montalban were all very recognizable characters to TV audiences in the 1960's.
I don't know if there is any explanation, but Karl Malden appeared in a number of western movies from 1960 to 1970; including the Nevada Smith, How the West was Won, Cheyenne Autumn, The Adventures of Bullwhip Griffin, Blue, and Wild Rovers. He did not appear in westerns before or after this decade (1960 to 1970). His performance in Blue is excellent. He plays the resolute character with strong convictions that he does so well.
Joanna Pettet was an underrated and under-performing actress. Directors never seemed to take this actor to her potential. She does a wonderful job in this movie, playing both a vulnerable woman and a strong lover. I found myself worrying more about Blue (Terrence Stamp) because of Joanne Morton (Joanna Pettet) character's love for Blue.
Blue (Terence Stamp) is the intense young man possessing an extremely dangerous skillset (he is a gunfighter and a killer). He is confused by his conflicting loyalties (his sudden love for a woman he has just met that opposes his gang affiliation).
My only negative comment on this film is Terrence Stamp's strong British accent that seems out of place for an American kidnapped by Mexicans.