The TOS fx guys wouldn't dream of making it blue, because that would foul up the blue screen process.
In the original fx, un-remastered, there's a shot of the Enterprise leaving orbit where the starboard nacelle has a jagged, wavering transparent section. That happened because white stage lights were hitting the blue backdrop and sending blue "spill light" onto the shiny metal nacelle.
It was a simple accident the ruined the shot, and the fact that they used it anyway says something about the time and money constraints they were under. I think it showed up as stock footage in at least two episodes.