@schwarzesmarken muv-luv started with "Muv-Luv Unlimited/Extra" and that is also a good start into the Muv-Luv universe. I think on the Vita it was the alternative version? Every Muv-Luv is a single string in the Muv-Luv string universes, hope I can state that correctly with my rustic English xD
Schwarzes Marken is situated in the Muv-Luv universe but has no connection to any other Muv-Luv story. At least from what I know ^^