Good users for Blue Flame?

Best girls Sharena and Anna.

Anyone you want scoring high in arena, but dont want to use a long cd special (galeforce/aether)

In my opinion, anyone can really use blue flame. Units that already have skills based on being next to allies use it best, and units that have skills based on being away from allies don't, but overall most units can use it well so just choose whoever you'd like.

It can be especially good for units with Legault or Navarre-like statlines: lots of speed, everything else mediocre except a lot of awkward HP. These units have a hard time finding a special to use to take advantage of their speed because draconic aura and bonfire and iceberg don't hit that hard given their statline. But even then it's awkward because high spd units without a lot of bulk tend to prefer player phase, and during player phase they might need to move away from allies in order to attack. So it's an awkward special to use well.