Well, here is my reasoning:
To start researching the answer for this question, we need first to discover how to identify any trait of a Belmont's soul. Not many leads are ever given in the series, but I believe I have JUST the perfect evidence:

See, Death can obviously observe an individual's soul, and it tells the player that its color is one important trait of the soul of a Belmont. Furthermore, it can be deduced rather safely that Death recognizes Juste as being a Belmont because he has seen this trait of his soul - its color - a bunch of times in the past. So, Death recognizing the color of the soul of a Belmont right away tells me that, amongst Belmonts, this is a persistent trait AND and inherited trait.
Having said that, I believe that in Harmony we can TOO see the color of his soul. And its blue, as the outline in his sprite shows. Gameplay speaking this blue outline is meant to make the sprite pop BUT Circle of the Moon did the same thing using black. It looks like, to me, that since they had to add an outline, they decided to make it a special color to reflect
something, and I think we can agree that it looks like an aura.
On some belief systems, the aura is believed to reflect the color of one's soul. In many of these cases, though, the color is a constantly changing atribute, not a fixed one, that gets altered as the person experiences certain emotions. But in Castlevania, this seems to not be the case, as the Sorrow games have shown. This is also the case in this very cutscene since Death manages to identify the color of Juste's soul even with him displaying a range of different emotions.
Beyond the technical reasons mentioned above (making the sprite pop out), one could argue that this blue outline merely reflects Juste's magic potential, and not in fact his soul - or a soul-reflecting aura. However, Maxim, who is NOT proficient in magic, ALSO has a colored outline. You can see it when you play Maxim Mode, and his aura is red:

Finally, Dracula Wraith ALSO has an aura of his own - yellow. Yet, his aura is a glowing one and not static like Juste's or Maxim's. This could suggest a number of things:
-The being/soul inside the being is not yet stabilized with the body, causing the aura to be unstable (and this instability is supported by the Wraith's own dialogue).
-It's not an aura representing the Wraith's soul, but something completelly different (an option I'm not too keen on, but also possible).
And that's it. My conclusion is that the soul of a Belmont is blue, and therefore is a Guardian Type soul. Of course, this conclusion means that Maxim's soul is a Bullet Type, and Dracula Wraith's is an Enchant Type. Curious how they reflect the main Soul Types of the Sorrow games, huh?