It's a single, uniform color throughout the feathers. So mine are not true "Self" Blacks because the cockerel has "leakage", another color that leaked into the feather color. My pullet is closer to a self black and would be a good example if I took her two white feathers out. I think Sunnydawn got some self black chicks from my eggs.
And no, it's not just in Polish.
Thanks for the explanation. I was asking because we have a blue silkie chick with some
leakage on her head, who wouldn't be considered a self blue...