Blue is ONE dilution gene, there are a number of them, including one called "dilute." DIfferent dilution genes dilute one or both plumage pigments (there are only two: eumelanin--black, and pheomelanin--red). THe dilution gene under discussion is the blue gene, designated Bl. It dilutes only black pigment. It is an incompletely dominate gene, meaning that one copy dilutes significantly less than two copies. Its alternative allele is bl, not-blue. Every chicken has two copies of these alternatives. A bird with two copies of bl has undiluted black in the plumage (ignoring other dilution genes); a bird with one copy (Bl bl) has somewhat diluted feathers. This "partial" dilution is called blue. A bird with two copis of Bl has significantly diluted black pigment in the feathers; this is called splash.
If one parent is splash: Bl Bl, they will always contribute a copy of blue to their offspring. If a parent is black, it will always contribute not-blue to its offspring. The offspring of these two parents will always have one copy of blue and one of not-blue: Bl bl. And this genotype results in the phenotype of a blue bird.
