Charcoal egg from Swedish duck?

I think my youngest duck laid her first egg this morning! It was a fairly dark gray egg, and because I wasn't expecting it when I collected eggs before sunrise, I missed it and it froze. My question, though, is that I have only Swedish and Pekin ducks, although I've been questioning the breed of one of them for a while. I've heard that Swedish ducks lay white or blue eggs, but haven't been able to find anything about any breeds other than Cayugas laying dark gray eggs.



These are the eggs I collected this morning. The egg in front is a light green chicken egg, but it appears white in the photo. The egg at top left is an off-white egg from my only other laying duck, and the gray egg at top left is the one I'm questioning.



In my flock, I have four ducks (a Pekin, a blue Swedish, a black Swedish, and a very lightly marked splash Swedish), and a Pekin drake. The blue Swedish and the Pekins were hatched in April 2014, and the black and splash Swedish were hatched in July or August 2014. I'd guess the black girl is roughly 3 weeks younger than the splash girl. I'd assumed that the grey egg came from the black Swedish only because I've made the connection in my head between black ducks (Cayugas) and gray eggs, but I realize I may be very wrong about that.

So, my question is: Can/do Swedish ducks lay charcoal-colored eggs, and if so, is the egg likely to be from my black girl, or can any of them lay such a dark egg? Also, I've seen so much conflicting information about what color eggs to expect from Swedish ducks, so if you have any experience with the breed, please feel free to share!