I would not use a bird of mixed breed to start a new variety. If you want a Blue Partridge, go with a Blue or Splash Plymouth Rock and a Partridge Chantecler. The cross would be better. You never know what a sexlink is carrying and would pass on to the next generation.
Take a Blue Rock male to a Partridge Chantecler female and a Partridge Chantecler male to a Blue Rock female. Take all the offspring that looks good for type and color and mate those. Then take that generation and mate back to a Partridge Chantecler. A new variety project may take 10 years, but it will be nice when it is done.
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What are these?
Their mother is a splash Wyandotte and their father was a NH/Black Sexlink.