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Don't worry about it.
It's just some discoloring the foil didn't prevent from a minute amount of O2, foil never prevents 100% discoloring, just scale and decarb. Did you sand it off before the temper, because the temper would have brought it right back.
Does the file dig into it at all? I usually tempered D2 at 425 for a HT temp of 1850 and that left me around RC 60 or a little higher and a good file should barely bite into it. A regular HS drill bit wouldn't drill into RC 56 either, just FYI, don't go by a drill bit test. If a good file won't bite into it it may be too hard and possibly brittle. I presume you know how to do a brass rod test? It won't hurt a thing to re-temper at 425 if it is too hard.
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