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The scraping that you did may have removed the finish but won't hurt the bolt at all unless you used a dremel with a grinding wheel or something.

It's just not necessary to get that area all that clean. The buildup is pretty much self limiting and an argument can be made that the buildup actually aids reliability by making for a tighter gas seal in that piston area of the AR-15.

One argument for getting that area perfectly clean is that buildup will hold moisture and could cause corrosion. I don't buy that personally. Pat Rogers who shot more ammo out of AR15s than just about anyone and ran them for tens of thousands of rounds without any cleaning at all stated that he had never seen corrosion on a BCG. That tells me that argument doesn't hold water for most of us.

Here is a picture of one of my bolts with 750 rounds on it (about the same as yours). The flat part of the tail always gets shiny. I usually clean with just some solvent (Hoppes 9) and a rag and call it good enough. You also see some light grease on there which is my lube of choice these days. ETA: This is after I gave it a light cleaning.

It's not wrong to get it perfectly clean and they make scrapers designed for just that purpose (BONE tool, Cat M4 tool, etc).

Or you can use an empty 223 case to scrape it clean, and that is brass so it is very easy on the bolt.

So, its not wrong to get it sparkly clean, its just not really necessary.

Yup.