Just wanted to add, Komachi was a child when she started making sense of her ability. Children are notoriously bad at making sense of things. After losing a loved one, kids will often blame themselves even if they weren’t involved in the incident. This can range from “he died because I told him to go away” to “he died because I didn’t eat all my lunch”. It’s important for parents to tell their kids that they aren’t at fault while they’re grieving. Komachi never had an opposing voice for her beliefs, so she kept believing it. In addition, there’s not much she can do to disprove it. She’ll always see death wherever she does, because death is everywhere.
It makes me think that her understanding of death, as well as her logical thinking process, is somewhat warped. Her logic seems to fall inline with magical thinking. She thinks that everywhere she goes, death will follow. She also knows that death is natural and happens to everyone no matter what.
As for why she thinks the black is following her in the first place, I’ve had a lot of kids tell me that the moon/sun is following them. Silly, right? The sun/moon is always there, it’s part of everyday scenery. It’s really far away, so we always see it. (Me trying to explain to the kids). But kids don’t really realize that. The same goes for Komachi seeing black. Kids, if the constant flow of “me, mine, my” didn’t tip you off, are very self centered. Everything feels very personal to them.