Should we tag the eraser as Hiei since it sports her color scheme (and of course her name)?

We don't tag with just features, we tag the characters themselves, I believe. That goes with lots of poses too.

The character's name + objectification or adapted_object, whichever fits better. I'd say the latter in this case since it's just her outfit pattern (but character-specific outfits do get tagged with the character's names, regardless if the character is present, as is the case with cosplay).