Well, I think I will still like my blue chickens, but I should have known to look them up BEFORE I got excited and bought them, instead of after. Apparently "Plymouth Blue Rocks" are not true Plymouth Rocks. They are a hybrid sex link breed, sometimes called Sapphire Gem.
https://dominant-cz.cz/produkt/dominant-blue-d-107/?lang=en
So it appears they are half Barred Rock and half Blue Andalusian. The good news though, is that since none of mine have a head spot, I probably have all pullets!
It cracks me up when they have a "straight run" bin of a sex-linked breed. I didn't see a single head spot in there. But I did try to feather sex them by their wings, and the first four I picked up (the ones I bought) all looked "pullet" to me. Now I know why!
Oh well, and here I thought actual Blue Rocks were making a comeback.
Maybe I should call the helpful lady at the RK and let her know what I found out.