In movies, Broken Arrow (1950) probably was the biggest breakthrough. Other milestones along the path to authenticity were Fort Apache (1948), The Man from Laramie (1955), The Searchers (1956), The Tin Star (1957), The Light in the Forest (1958), McLintock! (1963), and Cheyenne Autumn (1964).
On television, The Lone Ranger (1949) undoubtedly was the biggest breakthrough. I haven't seen most of the '50s TV Westerns, so I can't speak for them, but Northwest Passage (1958) had a semi-positive attitude toward Indians and Bonanza (1959) was largely positive.
The typical '50s Indians
McLintock!, which I recently reviewed, includes several of the transitional elements in the portrayal of Indians:
The '50s movies and TV shows began to challenge the idea of Indians as aggressors attacking the innocent white settlers. They began to acknowledge that Indians were the victims of the "taming" of the West. But the shows didn't go any deeper than that.
In particular, they rarely questioned why the Indians were suffering. Although the middle-aged protagonists played by John Wayne, James Stewart, et al. lived through the Indian Wars, no one was ever responsible for breaking a treaty, defeating an Indian, or establishing a reservation. Somehow these things just happened and became the status quo.
So the people in these shows began accepting and dealing with the "plight" of the Indian. What they didn't do was acknowledge their part in causing the plight of the Indian. That wouldn't happen until the late 1960s, 1970s, and 1980s.
For more on the subject, see The Best Indian Movies.