Nightmare Rarity might be Blue/Black
Rarity herself more likely mono blue...
as for Applejack, go big or go home!!
but you can do that with Blue/Green...
mystic snake with a Temur Sabertooth to return it to your hand works for a counterspell
add Murkfiend Liege to untap mana creatures and speed on to your endgame!
Kruphix for her commander maybe?
for an unending 'harvest' of mana!