I have BRs who are usually the speckled girls. Sometimes a RIR or Delaware egg will have speckles or even one of the Buff or Blue Orps, but it's not the same every time. I have a couple BRs who lay an egg as dark as the one in your picture.
here is my favorite speckled egg.. it has white specks on it! It is layed by a black Ostralorp. Which is weird because growing up mom had a lot of those birds and I don't remember any brown eggs with white spots.
I don't think it is breed related.
it is like the color of the eggs. Sometimes they are lighter and some times they are darker. sometimes they have speckels, more white on it and sometimes not.
I know which eggs belong to which hen and they all do it some times.