Real wires and cables are black because protection materials that resist UV damage the best are generally black. Older lead telephone cables, up to 3" in diameter, are gray. Newer "inner duct" which is used to identify optical fiber cables, is made of orange plastic, which fades to yellow, as the red pigment in the orange plastic is destroyed by the UV. Bare copper and copper-weld (iron wire coated with copper) wires used in open-wire construction for telegraph and telephone service was about 1/6th of an inch in diameter and would corrode to a light greenish tint. Those are the historic telegraph wires found along old railroad rights of way.
A 1:48 scale telegraph wire would be 1/6 x 1/48 of an inch. That's 1/288th of an inch, which would probably be difficult to see in any color.
I've never seen blue wires on poles, but I've also never seen a 20' tall crossing flagman, either.
