Emblem heartless are artificially created and pure blood heartless are "natural" results of a person's heart falling into darkness. As for why the distinction matters so much, there may be a hint buried in a Secret Report

Good question! Probs have one of each for collecting; but for gameplay, Keeba Tiger and Tatsu Blaze are hands down cooler in Nightmare than regular Spirit.

All the dinos, birds, Drak Quack, Electricorn would be cool to switch out based on the world atmosphere (brighter Spirits in PP or TT, nightmares in Grid and TWTNW, for example).

I feel like the Dream Eaters are underappreciated. Especially the color variation between Nightmares and Spirits. I kinda wish there was an option in DDD to allow a palette swap between the two.

Aura Lion is peak tho I'd probably be a Tyranto Rex.