Part of a Pullman porter's job was to make up the sleeping berths in his assigned sleeping car, and to provide extra blankets to those passengers requesting them. The standard Pullman wool blanket in the 20th century was dyed a salmon color, which became almost a trademark of the company. When a blanket became worn or damaged in service, it was assigned to those blankets reserved for porters' use.
Ostensibly to avoid mixing these with the passengers' blankets, the porters' blankets were dyed blue. (This was done nationwide, throughout the Pullman system, not just in the South.) A dyed-blue Pullman blanket is today extremely rare, given its negative racial symbolism.
See other Pullman and Pullman Porter-related objects and photos in this database.