MEL

mirrorSelectedObjects
This MEL script will mirror selected objects without inverting they're vertex normals. This should keep your poly objects from turning black when they're transforms are frozen.
Download from Highend3D.com

 

mirrorFixer
This MEL script will search for objects with negative scale attributes (the common way of mirroring objects) and fix they're vertex normals, preventing the objects from turning black when they're transforms are frozen.
Download from Highend3D.com