Sorry for the dipped out white coin fans here is one that escaped the dipping jar for better or worse. Its grown on me over the last few decades.
It doesn't matter. The final stage of toning is turning black. It may not go any further and it may not affect all of the toning, but it is still the final stage. Yes, it is possible to slow the change if it is slabbed or kept in an airtite. ~ Chris
since its been quite stable and in a slab for 20 plus years that is a risk I will take the toning is due in part to the slab insert, for a 15 buck coin it is not such a big gamble anyway.
Aesthetics are completely subjective. Collectors of modern silver mostly frown on black toning. Here's a Roman denarius that's gone completely, beautifully black...