If both those chickens are what they are supposed to be you will definitely get an Easter Egger. The rooster will give the blue egg gene to all his offspring. Since that gene is dominant you will get a base blue egg from any pullet he fathers. Since the Marans hen is supposed to lay a brown egg, any pullet offspring will almost certainly lay a green egg. Green is just brown laid on top of the blue. The base blue egg genetics are pretty simple as long as the parents are pure for that gene. “Pure” means both genes at that gene pair are the same.
Brown egg genetics are a lot more complicated. There are a lot of different genes that affect what shade of brown the egg will be. Some are dominant, some recessive. Some only act if another gene is present. One will bleach some brown genetics so you get a white or very light egg even if some dark brown genetics are present. One is even sex-linked, meaning the father will give it to his pullets but the hen won’t.
To further complicate it, not all Marans lay really dark eggs. You often see complaints about that on this forum. If the person selecting which chickens get to breed does not select for dark egg genetics, Marans eggs can get pretty light. What shade of brown does your Marans hen lay? If the Legbar rooster is pure he should not contribute any genetics toward brown eggs, but he might contribute that “bleach” gene.
What you can normally expect with that cross is a green egg. If the hen lays a fairly dark egg you will most likely get a fairly dark green egg. If she lays a light brown egg you will probably get a fairly light green egg. Beauty is in the eye of the beholder, so is shade of brown. What might be dark enough green for me to call an egg “olive” might not be dark enough for you.
I’ll add one further complication. If you hatch two pullets from that cross they may not lay exactly the same shade of eggs even if both have the same parents. It just depends on what genetics each happens to inherit. That happens to me all the time.
So you will get a green egg. Whether it is dark enough for you to call olive you will only know for sure after you see the egg the pullet lays. If the Marans hen is laying a dark brown egg, your odds are pretty good.