Mine is all blued. If my memory serves me correctly the earlier magazines had throats that were heat treated. This affected that areas ability to take the bluing. Later, a higher grade of steel was used for the whole body thus negating the need for the heat treatment. Again, I could be wrong.
The original Colt 1911 magazines were full blued, and were blued in the same charcoal furnaces as the pistols. It was found that the heat from the bluing process was taking the tempering out of the magazine lips leaving them too soft. Colt continued to use the heat bluing process, but tempered the magazines after bluing by immersing the top of the magazine in a molten cyanide solution and tempering in oil. The first of the magazines blued in this manner only had the first inch or so tempered, but the specifications were changed to temper them to below the magazine catch. The molten cyanide removed the bluing, but Colt did not find this objectionable as their commercial magazines were tempered in the same way.