I know how blue works. If you cross blue with blue you get 50% blue, 25% black, and 25% splash. My question is a bit more complicated. I wanted to bring some Blue looking ducks into my flock. My friend has some Blue Swedish that his kids are bringing to the fair. I was going to mate his Blue Swedish with my Black Swedish/Rouen cross. My Black Swedish/Rouen are black with a spotted bib. The crosses crossed with each other made an all black duckling. How many eggs would I need to get to make sure the combo gets a blue. My Swedish/Rouen is the female and my friend's Blue Swedish would be the male.
https://www.backyardchickens.com/t/410593/understanding-basic-colour-genetics-mallards-derivitives Old thread that covered duck color genetics. Think in your case the question is what are your black swedish / rouen crosses genetically. If you were just dealing with a black x blue swedish cross you should get 50/50 average of black and blue with a large enough number of ducklings, remembering that each ducking is an individual event... like flipping a coin.