Blue and Lavender (self blue) are two different genes. Blue mostly dilutes black and has a laced appearance, and lavender dilutes the expression of both black and red (eumelanin and phaeomelanin) areas of the plumage...but in reference to the lav orps results in an even light bluish grey.
Blue (Bb) (incomplete dominance)
Blue to Blue = 50% Blue, 25% Splash, 25% Black
Splash to Blue = 50% Blue, 50% Splash
Splash to Black = 100% Blue
Splash to Splash = 100% Splash
Blue to Black = 50% Blue, 50% Black
Black to Black = 100% Black
Self Blue(lavender) (recessive)
Self Blue + Self Blue = 100% Self Blue
Black + Self Blue = 100% Blacks, all carrying Self Blue
Black (carrying Self Blue) + Black = All black, 50% carrying Self Blue
Black (carrying Self Blue) + Black (carrying Self Blue) = 75% Blacks [50% carrying Self Blue, 25% pure Blacks], 25% Self Blue
Black (carrying Self Blue) + Self Blue = 50% Blacks [all carrying Lavender], 50% Self Blue