To help prevent this moment from being more common, use lots of various light sources. I have both incandescent and fluorescent work lights that I use to check scratches when sanding. I sand mostly under incandescent, and then periodically check with the fluorescent. I also take the blade out into the sunlight or even flat light under a window. I also get close to the blade when looking for scratches and look from various angles.
If you see a little bit of a scratch left, do not give in to the temptation to go ahead and move on to the next grit, thinking it will come out with that grit. It won't.
And when in doubt, keep sanding at the same grit. When you think you're done with a grit, go ahead and sand a bit more.
--nathan