This silly notion about historical inaccuracy..
as far as black people's presence and achievements during the American Frontier period of American history is absurd and intellectually dishonest. As other posters have tried to point out to the "Anti-PC" crowd who want to maintain the "historical accuracy" of there being little to no black people during the time of the Old West in regards to this Magnificent Seven remake, the truth is that The Old West was full of black cowboys, bounty hunters, criminals and men who led posses and gangs (on both sides of the law). The reason you never got to see these types of people portrayed in old Hollywood westerns is fairy obvious. In film history, westerns have downplayed the presence or achievements of non-whites. It's the nature of Hollywood.
Contrary to American History taught through watching western films from the 50s, 60s, and 70s, there was a substantial black presence in the 1800s out in the Old West. The people (lawman, outlaws, cowboys, etc.) below are just a snippet of that history.
Bass Reeves (1838-1910) - One of the first black deputy U.S Marshals in the American Frontier who worked in the Indian Territory and Arkansas. Arrested over 3000 felons and shot and killed 14 outlaws to defend his own life. Even tracked down and arrested his own son for the murder of his wife. Denzel Washington's character in Mag 7, Sam Chisolm, is based on Bass Reeves. His occupation in the film was a federal officer and bounty hunter who worked in Kansas and as a peace officer in the Indian Territory. Reeves is also believed to be the inspiration for the Lone Ranger because of his exploits during his career as a deputy. Due to his stature and experience as a lawman, Reeves did lead several tough, mostly white posses often in his career, and he was known and respected throughout the frontier during his heyday. The guy was a legend in his time and should be just as famous as Wyatt Earp, Doc Holliday, or Billy the Kid, but sadly since he was black, there was no particular interest to burnish his legacy with countless movies,etc. Hollywood had no interest, especially during the Western movie heydays in the 50s and 60s.
Cherokee Bill (1876-1896) - Real name was Crawford Goldsby. His father was mixed with Black, Sioux, Mexican, and Caucasian Heritage and his mother was reportedly 1/2 black, 1/4 white, and 1/4 cherokee. Outlaw who met and joined Jim and Bill Cook's gang, "The Cook Gang", when was 18, a white gang by the way where they committed bank robberies, train robberies, and murder. Then later formed his own gang and rode with outlaws such as Henry Starr and Billy the Kid. Again white outlaws letting this "mulatto" and his gang ride with them. Said to have murdered as many as 13 people during his time as an outlaw. He was caught in Arkansas and executed by hanging a month after he turned 20.
Ned Huddleston aka Isom Dart (1849-1900) - A black rustler and Outlaw who was notorious in the Wyoming Territory. Part of a notorious band of rustlers called "The Tip Gualt Gang", a white gang, and also trained horsed for The Wild Bunch gang, Butch Cassidy and the Sundance Kid's Gang, and he was a successful rustler. He was shot and killed by Tom Horn a we has exiting the front door of his ranch in 1900. http://horsehints.org/CowboysWorld/BlackIsomDart.htm (Link with a picture of the Tip Gault Gang on there.)
You have several other Black lawman during that period such as Grant Johnson (who worked with Bass Reeves), Rufus Cannon, Wiley Escoe, Zeke Miller, Morgan Tucker, Eugene Walker, etc who were not only part of law posses but also led law posses as well during their careers. Not to mention the Buffalo Soldiers that everyone commenting about the Old West should know about. http://www.greatblackheroes.com/wp-content/uploads/2013/01/bassreeves04.jpg
The Rufus Buck gang was a multi-racial gang of Creek Indian and Black outlaws, led by a black man. There were great black cowboys lost to history like Nat Love, who was one of the most famous black cowboys during his time. Mary "Stagecoach Mary" Fields, who was the second woman to work in the U.S. Postal Service and the first African-American woman. Not to mention the thousands of blacks during that time that were simply cow herders, ranch hands, etc. who just made their way in the Old West without having an "exciting, adventurous" life.
For further research and reading I'd suggest reading Black, Red and Deadly: Black and Indian Gunfighters of the Indian Territory by Arthur T. Burton, Black Frontiers: A History of African-American Heroes in the Old West by Lillian Schlissel, and The Black West: A Documentary and Pictorial History of the African American Role in the Westward Expansion of the United States.
Truth it seems, is not only stranger but more fascinating than fiction as well concerning African-Americans in the American Frontier (or Old West as people like to call it).