You called it right: I think it is soot. In other words, charred residue from food that came before.
I notice this a lot in two cases: when the skillet is not yet seasoned all the way, and when it has been seasoned but for some reason food sticks, then other food keeps sticking to that food until it forms a wide, shallow "hill" of char that looks like normal cast iron. If you can scrape a fingernail across the surface of the pan and get black soot under your nail, that's probably it. In that case I use a griddle scraper to tear it down then add another seasoning base layer.
The best tool I've found for preventing this is chain mail. It's stainless steel links that are softer than the cast iron but harder than whatever you're scraping off. It leaves a smooth surface behind and is really fast to clean a pan. Here's a link to more info. I don't have any experience with this company's products but the explanation is good.
Here's what over-easy eggs look like when cooked in cast iron at my place.
It's like any cooking: You get out what you put in. If you put soot into your recipe, expect soot to come out of it. In other words, if your pan has charred food in it before you start cooking, you can expect it to get transferred to the food. If anything gets charred in my cast iron skillet, I clean it thoroughly before cooking anything else. I'm not a fan of soot or charcoal in my food.
If that was my pan, I would scrape it vigorously to remove the charred food. Perhaps even scraping while vinegar or water is boiling in the bottom of the pan. Then scrub it vigorously with a stainless steel scrubbing pad to get back down to bare metal. Then start over on seasoning the pan. Light coat of flax-seed oil in the oven at 350 F for an hour is typical in my home when starting a new seasoning coat for cast iron. The stainless steel scouring pads that I use are sold at all the local grocery stores. Sometimes I see copper pads which are similar.
On a sorta related topic, the cooked egg in the original post looks like it was cooked on very high heat. Cast iron cookware seems more suitable to me when used on medium to low heat. The pan's seasoning gets burned by high temperatures.
I typically rinse the cast iron pans as soon as the food is served. Perhaps with a light scrub using a stainless steel pad. Food rarely sticks to them when properly seasoned and used on low/medium heat.
I've heard that at least some burnt/charred food can be carcinogenic, so I would try to eliminate the charred specs. What I normally do with cast iron skillets is put just enough water in it to cover the bottom as soon as I dump the food out and return it to the stove to let the remaining latent heat warm the water. I let it soak while I eat, and it usually cleans right out with a brush. If something really dried out in it, I let it soak overnight (filled with water). If it's really bad, bringing the water to a boil before scrubbing usually takes care of it.
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