You get one per best of 3 win at astral level 5 (5 stars). My current prioritisation is dark celestials (because you can't get them in packs) > very rare cards you can get in packs (esp. fusions) > others.

Originally posted by TM2:

Levelling up seems far too limiting

Dark Cards are supposed to be end game cards, that's why you only get them with leveling up. But once you hit an Astral Link 5 with a character, you can spam the best of three battles like Volandum said. Each time you win you get the same reward as leveling up.

Long story short, at level 5 each 2 wins or so give you a Dark card.