Actually it has nothing to do with the inhibitor gene, which is the gene responsible for smokes.  If a calico does not inherit two copies of the recessive dilution gene (one from each parent), she will be a dominant calico or black, red & white.  If she inherits only one copy from one parent, she will be a dominant calico that carries dilute.  However, if she does inherit two copies (one from each parent), this dilutes the black & red coloring to blue & cream making her a dilute calico.

Exactly Jaycee.  Blue-cream is simply the diluted version of tortoiseshell just as blue is the dilute of black, cream is the dilute of red and lilac is the dilute of chocolate.

No, I wouldn't say dilute calicos are rare.  In fact, I have just as many if not more dilute colored kittens as dominant colored ones.  And the majority of my Persians are dominant colors, carrying dilute of course.  As long as both dominant colored parents carry dilute or one parent is a dilute and the other is a dominant carring dilute, you have good odds of having some dilute kittens.  And breeding two dilute parents will always give you only dilute colored kittens.