Can bickslow see the dragon's soul, or do the slayers own soul block him from see them?
Great question! The short answer: kind of. The long answer? I'm using your ask as an excuse to attempt to explain souls as Bickslow sees them, and his magic, because Bickslow doesn't ever want to do full breakdowns of it in his pov lmao, and also since it's hard to explain in general, by design. (I have a oneshot coming that gets close, but just in case Bickslow keeps fighting me, here's the infodump with visuals lol.)
So. Bickslow doesn't always see souls: he has to look, but it's very easy for him to look. Sometimes he does it on accident. It blinks in and out of vision and sort of blends together for him. If he's not trying, it's all very faint anyway, obscured by the flesh and whatnot.
Every soul is very unique. They have their own mix of colors, their own textures, their own patterns, and they move. With emotion, with change, with just being alive. They're wriggly things like that. This combination does mean that there are very few "rules" for how things are supposed to look. Doubly so when magic gets involved, because that also gets reflected in it. Different species have different "shapes" in a way, and that's solely because of how they interact with their own bodies and they're magic. The soul is the essence of character, sure, but it's also life magic. It's all of the above.
For example, humans—which is the default for Bickslow's magic, but he can see and to an extent control anyone now thanks to the uhhh *ahem* conditioning—have a very basic structure. They are flexible above all else, and they're magic doesn't really affect their bodies as much, so Bickslow would normally see the soul essence concentrated in the chest, the hands and feet, and eyes. Pretty standard for everyone. Demons are similar but theirs also spread out a little more, because their bodies hold more magic and in turn is more affected by it; their core areas are connected by more prominent areas, following the magic channels.
For mages, it gets extra complicated. In the diagram above (I was lazy with these don't hold them as *too* canon or anything), the lefthand side are non-mages, so their soul qualities are purely reflective of them. On the right, we have... let's say ice mages. Their souls are brighter, with brighter spots, solely because of magic, but also those spots may look like little ice flecks—snowflakes and icicles and frozen surfaces. The more they practice, the more it reflects.
History can also reflect. Physical scars, emotional scars...
(Erza was such a bad example to use because her armor effect makes it hard to see lol, but I committed.) Not every scar is reflected in the soul, for instance. In Erza's case, ones she received during her stint being literally tortured by cultists (and thus "causing" Jellal to trade and go "crazy") cut deeper than some scratch by a wild animal. It varies.
As for dragons... well. They are very magic, and thus very dense. Their core areas are nearly indistinguishable, because they (like celestials/spirits) have a very direct relationship between soul and body.
Bickslow might not have known what Acno was at first, but he knew he was big. Or very powerful. Or both. (The answer was both.)
Acno has to condense himself into his human body, and it works, but he's not sacrificing his dragon body either, so sometimes Bickslow can see the flare of where the rest of it should go. (Which, sometimes, really powerful people also have that bigger than life flare, like Gildarts. It's the literal aura, and whatnot.)
So, for a dragon that, say, pretty much sacrifices their body to completely condense themselves? In another soul? A bit tricky to spot.
Bickslow can mostly notice because they are extra dense and move differently, but dragon slayers are like mini dragons, although with some base species in there too, so there's a lot going on. Now that Bickslow knows they're there, it's more noticeable, of course, and even more so if they become more active and Bickslow can see the big imprint behind the very smol dragonlings. These things certainly don't come with a visual manual, though, so Bickslow spends a lot of time guessing and ignoring. Especially when it comes to *wild* bodies like whatever Natsu is. And it's all on fire, because he's a fire guy. (I drew the fire badly because I was too lazy to change brushes, shh.)
So uh. Yeah. Bickslow can see them. Bickslow sees a lot more than he tells people, usually.














