According to Holderread, a blue/blue mating produces 25% black, 25% silver, and 50% blue (it sounds like maybe what Holderread calls "silver" is what you mean by "lavendar"...?). I'm recently familiar with this because I just hatched out some blue/blue runners (one black and one silver, as a matter of fact, lol).
Storey's guide does list "lavendar" as a sub-type of ancona, where the genotype is E/e, Bl/Bl, d/d, S/S, R/R (One extended black gene, two blue dilution genes, two brown dilution genes, two bibbed pattern genes, and two runner pattern genes). Only the E/e, Bl/Bl, d/d would affect the actual color--the other two are pattern genes that produce the ancona's patchwork quilt look. So a runner with E/e, Bl/Bl, d/d might be considered lavendar then?
Now that that's clear as mud, lol, I'm going to sign off and go eat some dinner...