Lavender is two Blues producing only Blue offspring and no Splash or Black?
It's a totally different grey coloring gene than blue. It doesn't even produce the same shade. Lavender is a recessive gene where blue is incomplete dominant. Yes lavender bred to lavender only produces lavender. There can be a feather quality issue associated with the lavender gene, so black birds are also used occasionally in the breeding to improve feather quality.
Both blue and lavender dilute black. Lavender dilutes red as well.