First things first you will have to find a large self blue bird to mate with the dark cornish. The body type of the cornish is incompletely dominant over a body type like in a standard chicken. The offspring should have a similar body type as the cornish. This was my experience while out crossing cornish to other breeds.
I would start with a male dark cornish that has excellent body type and mate that with a standard self blue bird. produces F1
Each of the F1 offspring will carry on self blue allele. Then do an F1 x F1 cross and produce F2 - 1 in 4 of the F2 offspring should be self blue but they will be leaking some red color. You will have to select for lavender birds with the least amount of red showing and the best body type. Keep crossing the F1 birds until you get some good F2 birds. Then cross the good birds and pick out the offspring that are the best self blue birds. Always keep in mind the body type of the birds- if you are not careful the body type will be lost.
If you can not fix body type in the offspring by crossing the sisters and brothers, then once you get the self blue phenotype back cross to the superior male this will produce BC1 offspring and then do an BC1 cross to fix the self blue color with better body type.
Leg color may be a problem. Cornish should have yellow shanks and feet. What you do depends on the lavender bird you use to cross with the cornish.