I'm immediately in over my head when it comes to genetics. Aside from the variability that guppies show (where blues every now and again throw greens or purples) do you know if your guppies were carrying only the same color gene?  That one generation was blue, the parents were turquoise. That would indicate something is not all the same genetically.

I would bet some of your guppies have carried yellow all along. If you had a number of youngsters grow up, you might even be able to trace that by simply counting how many were what color. If the first generation were all one color but there was a 25% variation in the second generation, pretty clearly on of the genes (if there were only two) was dominant over another. When the next generation was mated one to another, a group of 25% with a different tale would clearly indicate a different color gene was possessed by one of the grandparents.

For an more detailed explanation of this, read

Basic Guppy Genetics 03: Mendel’s Laws and Punnet Squares

in the Guppywiki, mentioned below.

Now do you know for sure that the grandmother to your turquoise male has exactly the same color genes that he had? Were they guaranteed turquoise?

What hue was your blue guppy? There are deep blue ones, light almost gray blues, metallic blues, blues with other patterns worked in and (I'm sure) a lot more. Just do a Google search for "blue guppy" and you will see what I mean.

Did you identify your "Japanese blue guppy" from a photo? If so, which one? Googling Japanese blue guppy gives a remarkably varied crowd of images. You probably could say that such and such a male resembles a certain photo labeled Japanese blue guppy. You only have a right to claim that your guppies are of one of those lines if your can document that the line is descended from blue guppies from Japan. Is that what your seller called them?

G'Ma shared a site which has described a lot of work on genetics. I have spent enough time here on Guppylog and with other things that I haven't had the time to go to "The Guppy Designer" at http://www.guppyinfo.com/

One must now have to register to use the site in depth. Some articles are available to visitors without registering.

You might also find G'Ma's observations useful:
http://www.guppylog.com/story/2006/12/21/133833/49

The owner of that site donated his Strains Library
to
http://guppywiki.org/index.php/Main_Page
That may be of use too.

I hope those are helpful to you. Please let us know what you find out about your guppies.