I think I was 16 or 17 and a pretty good seamstress by then. I remember looking and looking and not being able to see any sign of a zipper.
The zipper above (white dots on blue) is really well done. You can see it is a red zipper because of the zipper pull at the top, but you can't see a trace of the zipper in the seam. Go to the blog Behind the Hedgerow to see the original image and read the post that goes with it.
I tried to find out some history about the invisible zipper without any success really. It was introduced to the home sewing market around 1968 and used by the sewing industry as early as the 40s (I think). Please if anyone knows better leave comments!
As Carolyn in her blog Allspice Abound tells about.
BUT, if you are putting a zipper into a dress where it will be under some stress, such as the blue evening dress above, get a YKK brand zipper. These can be purchased in load of different colors at WAWAK.
When I made my daughter-in-law's wedding dress a few years back, I asked my sister, Theresa LaQuey, for zipper advice. Theresa does many, many brides every year and she recommended the YKK zipper for reliability in a close fitting garment.
So, if you've never used an invisible zipper ... give it a try. I think you will like them.