I see that this thread is a year old and the OP has been gone for four months anyway, but it just bugs me that in the thread's original run only a single post actually answered the question the OP asked, which was about psychedelics and not dissociatives (and his posts show that he is well aware of the difference).
For me, DXM does feel sort of like what I would call "dark", but I feel it's really more just like neutral and strange, like I have seem some alien things on it but I've never encountered demon entities or anything like that. I have encountered them on 4-AcO-DET, and that along with 4-HO-DET and 4-HO-MET just give me a lot of skeletal imagery in general. 4-HO-DiPT and LSD also share a quality I get with them that makes me feel sort of demented or crazy in a way that is somewhat dark, but the hallucinations and everything really aren't. Lastly, DiPT gives me a very unique form of dark visuals with tentacles and vortices and the like, but the emotional vibe really didn't feel dark of crazy to me at all.