I’ve read that the black copper marans are supposed to have the darkest eggs.
I'm not sure why people think there is a genetic connection of feather color to eggshell color. There is not, those are totally different gene pairs. If you are concerned with feather color you look at one set of genes. If you are concerned with eggshell color you look at different genes.
You have an interest in feather color, specifically the B/B/S feather color gene. That is pretty straightforward. You generally know what you are getting.
Eggshell color is harder. The person that decides which chickens get to breed has a huge effect on that. Regardless of feather color if they use eggshell color as a criteria they can produce the genetics for a dark egg. If they do not use eggshell color as a criteria then you generally don't get the darker eggs.
Roosters don't lay eggs so you can't tell what eggshell genetics a rooster has by looking at his eggs. If you know what shade of eggs he hatched from you have a clue of what he might have, but he got half of his eggshell genetics from his father. That is a clue, not something definite. If you know what color of eggs his grandmothers laid you have more clues. Still not perfect but better.
Chickens inherit traits from both parents. But unless you know the genetic information for a few generations it can be challenging to know exactly what they have. Generally if you are looking at male traits in the offspring it is best to look at the fathers and not the mothers. If you are looking for a female trait look to the mothers.
You are looking for eggshell colors. My suggestion is to get Marans hens so you can see what shades of brown they lay and hatch the darker ones. Get a rooster that has the blue egg gene so you can go for olive eggs. That could be a cream legbar, Ameraucana, or others. Basically the opposite of what you are proposing. You can look at feather color on both male and female, you can readily see that.
So I’m wondering if maybe I should get a blue maran rooster now because then I’ll be able to hatch all three colors. What are the blue maran eggs looking like? Pretty dark? Just wanting some opinions on what you’d do. Should I get a black copper, splash, or blue rooster?