Are you certain that she's a Plymouth Barred Rock and not a crossbreed? She certainly doesn't look pure, by body shape. More like something a bit heavier, like an English Orpington.
If she is a crossbred, then she might carry a pattern gene that is not extended black (the dominant gene that makes black chickens their solid black colour. Genetic shorthand is E). That would give her chicks a 50% chance of not inheriting extended black at all, and thus, possibly not being black (depending on what pattern gene they inherited from her instead).
No matter what, if she's the only possible mother, and she's barred, then her male chicks will be barred. Which means that they will have headspots. Since they appear a very dark colour in that picture, you should easily be able to identify a headspt.