I am planning on doing black forest nightmares on my one account 

my question is after i do my first breeding of Black Forest horses x nightmares  which will in turn give me my Black Forest nightmares , for the foals can I pick either Black Forest horses or nightmares to do the 2nd breedings and either will result in Black Forest nightmares? So essentially Black Forest nightmares x nightmares or Black Forest horses x Black Forest nightmare will result in Black Forest nightmares?

I hope this makes sense

Yes that is correct!

Like doing a Golden Retriever x Poodle to get a Goldendoodle. I can then breed a Poodle or a Golden with the Goldendoodle pup and still get a Goldendoodle.

Thanks so much for the info!