Copied from
their blog today:
Negro League Stats: 1903-1948
We are very pleased and honored to announce that we have added Negro League Statistics from 1903 through 1948. These stats are due to a pair of collaborations. The first is between Baseball-Reference.com and the National Baseball Hall of Fame and Museum (1920-1948 data), and the second is between B-R and Gary Ashwill and Scott Simkus of Outsider Baseball (1903-1919 data).
The details of what is and is not included is understandably complicated and the list of contributors very long, so we have provided a detailed rundown of the stats and the people who researched them.
Special thanks also to Ted Turocy who helped immensely with the integration of the stats into our existing databases.