Naturally Toned Mercury Dime?

I agree the present toning is natural.

But I'd be a bit more specific. I suspect the coin was toned black, or nearly so, (terminally toned in other words), and somebody decided to dip it. Now either the heavy toning had already destroyed most of the remaining luster, or it was over-dipped a bit and that destroyed most of the remaining luster.

Personally, I believe the first is the more likely scenario. Why ? Because of the dark toning that can be seen around the rim and outer edges of the fields. When you see the remains of dark toning like that where it's been partially but not completely removed, coupled with basically blast white surfaces on most of the rest of the coin - that's a pretty good indicator the coin has been dipped. It's also a pretty good indicator that the heavy dark toning was the culprit that destroyed most of the remaining luster.

It is by no means an absolute that this is so but it is highly likely given the present condition.

There is one more thing that these inferences indicate, the coin was probably not harshly cleaned. Sure, most of the luster is gone, but more likely because of the terminal toning - not harsh cleaning. Yeah, the end result is basically the same, but it can be an important distinction. Why ? Because it would mean the coin could still be cleanly graded. Assuming of course that the graders would be of the same opinion.

Things like this can sometimes help us understand what the graders do and why they do it - or don't.