If all you want is "pet quality" dark egg layers, yes. If you want Marans that conform to standard i believe it would not be an acceptable cross. Maybe someone can help with the "what do you get out of them" part of your question. I would assume a non-standard, non-accepted colour. I posted this question once before and I was told there was no good reason to make this cross.
You get mixed up colors. It causes a lot of problems. It has been done by mistake in the past and several lines are dealing with it now much to their dismay.
I think Karen has some good points here. If she is your only hen and want good egg layers, then breed her. Otherwise, perhaps locate a wheaten rooster, or trade with someone.
Are you sure your Black rooster is a Black Copper and not a Wheaten roo?? A lot of people confused the two, hence the Wheaten sports pop up with supposed Black Copper breedings. The roo in your avatar, tends to show the lighter hackle coloring which may indicate a "Wheaten influence", like a cross, or he may be a Wheaten, hard to see in the pic. Can you post a pic of your rooster? Does he have a brown (cinnamon) wing triangle?
