I think once the link between the blue egg gene and the pea comb gene is broken then there is no more link. I think if you crossed a Legbar with a brown egg laying pea comb bird there would be no correlation between blue eggs and pea combs.yes I’m familiar with genetics. I was saying in terms of longterm selection. This is assuming of course that you are ok with pea comb birds. And yes, in this case it would only tell you that most single combed birds don’t have the blue egg gene in the specific crossing program I mentioned- the pea-combed individuals would be homozygous or heterozygous in subsequent generation.
You could also do a cross with the opposite linkage such as something like Silkie x Cream Legbar. In this case the blue egg gene would be linked to the single comb gene and you would be able to easily select single combed individuals in F2/F3 etc generations who should also be mainly pure for the blue egg gene. Two things with this cross- 1) silkies’ level of fibromelanosis is not as much as Ayam Cemani 2) silkies have both the pea comb and rose comb genes. Rose comb is not linked to the blue egg gene so rose combed individuals would also mainly lay blue eggs, but it would still be easy to select for single-combed individuals.