The recommended breeding combination is Blue and Purple, though I haven't tried it. Read more on the Night Dragon's wiki page.
I got a night dragon by breeding Magic (lvl 5) with Athletic (lvl 7)
I kept breeding my Water and Magic Dragon, and got it eventually. Just keep at it, and check the breeding time at the beginning. If it says that it will take 23 gold to complete immediately, you'll have a Night Dragon coming your way.
It's a real itch to do. I got a night after a week, but haven't been able to get any other night-hybrids in over a month.
All nights attempts result in coral. Pink leads to love. And mythic/easter leads to firestorms. I have 117 dragons and maybe 20 that aren't the aforementioned breeds.
Night Dragon is Water Dragon lv. 7 and Magic Dragon lv. 7. It only takes 1 try! But it takes 23 hours...