That calls for speculation. If it is the Blue from B/B/S, one of the parents would have to be Blue or Splash. The B/B/S gene is a partially dominant gene, it’s kind of weird. If both genes at that gene pair are B/B/S, you get Splash. If only one is B/B/S you get Blue. If none are B/B/S, you default to Black. But that only affects feathers or parts of feathers that would have originally have been black, it does not affect other colors like red, buff, or white.
Another possibility is that it is not Blue but instead is Lavender. Lavender is a recessive gene that behaves like you would expect a recessive gene to behave, none of this partially dominant stuff. It is also a modifier of black, it does not affect other colors.
With chicken color and pattern genetics there are always exceptions and other possibilities. There are a lot of modifiers that I don’t come close to understanding how they work. But from what I know the logical conclusion is that both parents have Lavender if neither is showing Blue or Splash.
