Congratulations on the 7 babies! That's really good for shipped eggs, especially when they are so dark you can't see anything when you candle. I don't know anything about Marans, so does their color genetics work like Blue/Black/Splash in other breeds? I have Ameraucana that are B/B/S and I know I can get percentages of each color depending on what colors I match up. My first batch this year was perfectly matched up to this chart, it was kind of funny.
Black x Black = 100% Black
Black x Blue= 50% Black and 50% Blue
Blue x Blue = 25% Black, 50% Blue, 25% Black
Blue x Splash =50% Splash and 50% Blue
Splash x Splash = 100% Splash
Wheaten/Blue Wheaten/Splash Wheaton work the same way in Ameraucanas. If it is like that in Marans maybe that explains why you have both colors.
I woke up to several pips, and I just went and checked because I heard lots crying, and I just had my first baby hatch! Looks like it will be a blue roo. Hard to be sure at this point, but the comb is so wide and it isn't dark enough to be black- I think.