As someone else said, a black chicken cannot "carry" the gene for blue feathers. If a chicken has the blue gene, they show blue feathers (one copy of the blue gene) or splash feathers (two copies of the blue gene.) A chicken that looks black must be pure for not-blue. The blue gene affects all black on the chicken, but not red or gold shades. So it turns a black chicken blue, or a Black Tailed Buff chicken into a Blue Tailed Buff chicken, or a Black Laced Red chicken into a Blue Laced Red chicken, and so forth.
Lavender (also called "self blue") is different in two main ways.
One, it is a recessive gene. This means one copy of the lavender gene has no visible effect, but two copies of the lavender gene do have a visible effect.
Two, lavender affects both black and red. So black becomes lavender (light gray) and red becomes a cream shade. Examples: Black becomes Lavender, Mille Fleur becomes Porcelain, Black Breasted Red becomes Isabella, etc.
