We used the light blue Phelps-Dodge blue cable well into the late 1970's in new work. A lot of their cable was also black. What was odd was that in many cases, the black conductor was actually brown. I don't know if this was intentional or just a case of someone not mixing enough dye into the batch.
We also used a lot of cable made by Cadillac Cable that had a green or brown jacket.
I'd say that it's safe to guess that it was just a case of manufacturers wanting their product to be more identifiable.
