Will my BCM still lay DARK eggs if the roo is a Buff Orp?

It does not matter what the roo is, her egg color is dependent upon her genetics, not the genetics of the egg.

Any offspring, however, will lay eggs in color somewhere in between those of a BCM and the father. If you are going to try breeding her, I would suggest putting her under an Easter Egger roo, as that would produce Olive Eggers, which are rather popular at the moment, much more so than a  BCM x Delaware cross.

yes, every hen's egg color is programed in her when her parents mate - her mate has nothing to do with her egg color, just her children's egg color.

you hen and roo's daughters will have lighter color eggs... your hen and roo's sons will not lay eggs.

Right, you got it now!

Now remember, 'painted' eggs (brown specifically) will get lighter as the laying season goes on, but after moult in fall the eggs will be dark again.

i was under the impression that the male determined egg colour

and due to BO having a pinkish shell colour the BCM offspring would have a light brown egg

Another thing to remember with the Marans is that they don't all lay super dark eggs. If you want the really dark ones, you have to make sure you're getting a hen that lays that color. 

im  a little confused to as i read somewhere cant remeber where but will try find it that the MALE determins the egg colour

so what i understand is if you Cross a BO MALE with  a BCM FEMALE

The eggs will be more towards the colour that the male breed has

considering BO Hens lay Pinkish Eggs the Chocolate Brown of the BCM will be diluted in the offspring of the pair

for example

the White Star Hen is a cross of a leghorn Male and another breed im trying to find out and so i get Porcaline White Eggs from Her

The Amber Star is a cross with a RIR Male and Leghorn Female and hence i get Light Brown Eggs