Hi swan!
Genetically we don't know the genes involved to produce this elusive color. There are some anecdotal theories concerning the graying gene, but still we don't know for sure.
Even if we did, there are very few people who really know the basics of genetics.
Furthermore we need honesty.
The majority out there enhance the true color of their dogs. And this is not something new. It is recorded through early history. It was and still is a common practice.
According to this theory and to answer your question, it is not that simple.
Breeding together 2 blue dogs the resulting pups may have the correct texture but they will probably lack in uniform shading. Some will be darker in color and some lighter in color.
To solve this throwback many employee a woolly or cottony or a mix textured black dog.
Of course this is wrong, because you introduce the wrong texture into the gene pool.
Old timers used the so called "red legged" yorkies to fix color in their lines.
I really don't know if these dogs had wrong texture or they just had an abundance production of melanin.
Some very interesting articles:
Color and texture, by Joan Gordon
Color & Texture by Joan Gordon
Genetics of blue/tan coat color, by Cher Hildebrand
Goldenray Genetics of Blue Tan Coat Color Yorkies Coat Color