black dragons seem a bit too ambitious for dmg..
i would suggest a healthy amount of cards
30 is the minimum and the optimum amount of cards for a deck, however not all decks need to heed this rule
I haven't ever considered a fire/dark duo deck, but I think it might work together well.
I think adding dark for utility mostly, like more cloaks and steals. get rid of photons and immo your devourers instead.
Instead of having golems and dragons, why not use fire spirits? cloak will keep them shielded while they grow, and fire spirits can grow to extreme damages when they're invisible from cloak.
In short, use darkness for cloaks, and take out the creatures and add in fire spirits. maybe keep golems? I don't know; this probably needs more testing. devourers as immo fodders. and you don't have to follow the 30 card rule; if you feel like you need to add more cards for more
and probably cloaks, do that.