In fact, I'll feel and hear the point where the blade no longer cuts the lump as I scrape over it
As you can see in reflected light, I've scraped the filled spot completely even and flat:
My fill now stands about 1 mil above the surrounding finish. It should be relatively safe for me to sand with a hard block and very fine paper to level the spot and make it ready for polishing.
Scraping right into the very corner works the same way, but with the entire blade protected with tape except for the tiny cutting end edge:
This way it's no problem to scrape into the tiniest corner without gouging.
At the edge of an instrument, it's the same deal. I have the long surface of the blade covered with tape to stabilize my cutting action:
I hope this little blade trick is helpful to you. It certainly was a major breakthrough for me!