If I may throw my two cents here, I considered the possibility of it being busoshoku right when I saw it as well but I'm inclined to believe it's not. The demonio form includes horns and vampire like canines, those have to originate from somewhere. Now, whatever the pwoer up that brought those about is, could Robin have achieved that AND learn busoshoku? Certainly. But since there must be an explanation for those, I personally assume it will encompass the form's color as well. As for what that power up actually is, I'm leaning towards Robin being close to awakening like Sei suggested. Well, still not sure about that but as far as the appearance of the new form goes, I don't think there are separate explanations for the horns / teeth and color scheme.
Personally I think we can attribute the horns and fangs to something more like Zoro's Asura than Haki. Soul stuff has always been spooky in One Piece, and while I wouldn't necessarily put it past Robin to use her power at a micro-level to create those features on herself, I also don't think we need to infer a fully coherent in-universe explanation.
The language she normally uses is along the lines of "the devil comes out of me", which implies that it's something she has within her that "comes out" at certain times, and here it seems to have "come out" so strongly so as to change her physical features for a moment. Again, I don't think we should restrict the mechanics here to either be Devil Fruit or Haki - if we never get any such explanation, then I think the safest bet is to assume it's soul-work (since that restricts its usage to just Robin without complicating any other mechanics in the OP universe).
Along the same lines though, I really think Oda has sort of ruined Robin by this stage. We've always been told that she grew up working as an assassin and spy for various organisations. She even made that clear when she joined the crew. However, we never see her think or act like an assassin. It's a little more nuanced than what I'm making it out to be, but I really feel that she's an impostor walking around with Robin's past, but isn't really Robin. Like, we don't even have a frame of reference by which we can say "yeah she's really got a devil inside her when her friends are in danger".
She has had a few wholesome character moments in the lead-up to the Black Maria fight (specifically with Sanji), but Oda just forgot to make her vulnerable ever since her "I want to live" moment. We don't really see her get impassioned about anything - she is the archetypical stoic, dignified, always-got-it-under-control type woman (Merlin from 7DS is another great example of this) - but where these characters have limits that make them distinct and human, Robin doesn't seem to have that. At least, not in a way that feels human - to this day I still feel like Oda is working out what to do with her. The lesson she learnt back in Enies Lobby was that she should rely on her friends, yet to memory we have never seen her humble herself and ask them for help in a meaningful way. We never see her try to connect with them at all; she just hangs out by herself and reads. We don't even know what it is that she's reading all the time - is she learning something useful in her reading, or is it just leisure (like fiction)? Enjoying reading is a common trait, but it is made unique by specifying the kinds of books that are enjoyed as they align with the characters other interests and goals, but here she is just reading, 24/7, with no in-between. If they are archaeology books, what information is she gaining from them? Wouldn't such books likely have clues about their Poneglyph goal?
Idk, maybe I'm way off-base here, but the best way I can describe my feelings on Robin is that I want to like her and empathise with her struggles, but I feel like I have nothing to connect with.