From these angles, the tan/brown coloration is minimal, so I think it was an artifact on the first picture. So she looks like a Lavender in these pictures. Lavender and Self Blue are the same thing. It is just one of three colors that you get when you breed Blue Slate to Blue Slate. The Blue Slate genetics are more complicated than other colors, in that purebreds come in three different colors: Slate, Self Blue (aka Lavender), and Black (but not genetically the same as the Spanish Blacks, just a black colored Blue Slate). So she does look like a purebred Blue Slate, just in the Lavender color, which is probably best for the colors that you said you wanted to develop, since you'd be starting one generation closer to that goal.
If this is making your head spin, check out the Blue Slate page on the Porter's website, then look at the feather color genetics page. If you go back to it many times it will make more sense each time until something will "click" in your brain, and all of a sudden it'll be obvious.
Until then, think of it like the three different colors in Labradors: Black, Chocolate, and Yellow. (The Labrador genetics are actually a little more complicated, but most people are more familiar with the three colors in them than in Blue Slate turkeys.) All three colors are purebreds, and often the puppies aren't the same color as the parents, but they're still purebred labs.
Cool!! Thanks!!!