I'm not talking about black coins but maybe dark charcoal gray in color and earlier coins such as draped bust and capped bust or maybe even liberty seated and barber coinage. Does it take away from the value? Would you buy a dark coin if it was priced right (to keep, not to flip)?
I'm less concerned about a coin being darkly toned, than about it being unnaturally so. Many coins have been abused over the years, and darkly retoned to hide the sins of their abusers. The coin below looks both unnaturally dark and suspiciously uniform in color throughout most of its surfaces, implying an accelerated oxidation of the coin, and has me questioning why. This coin was likely immersed in an oxidizing solution or impressed into an oxidizing substance. I could go on about other things that look wrong with it, but this discussion is all about depth of toning (credit to eBay seller xpojojx_8 for photos). The coin below is dark as well, but looks much more like I'd expect for a coin which has lived a normal life (credit to eBay seller barber for photos). Note the areas that are inherently protected from oxidation are lightest . . . the highest points not only tend to have toning worn off of them, but also often contact another surface, slowing oxidation of those points because less oxygen is available per unit area. This coin likely rested upon and was covered by oxidizing materials (sulphur-borne paper, leather, etc). Finally, the coin below not only looks completely natural, but exhibits what I consider extraordinary appeal for a darkly toned coin (credit to eBay seller redhouse_rarities for photos). This coin is more likely to have been surrounded by oxidizing material, rather than resting upon it, as evident from the depth of color in the peripheries and the gradual lightening of the toning toward the centers of the coin. It takes time for toning to progress radially as was the case here, and this is an unusually appealing example of that fact. I have had some darkly toned coins that were my absolute favorites for the date, hands down. I have also passed on a great many dark coins because they just did not look the part. Know the difference, and use it to your advantage.
Like a whole lot of things in life, it's basically a chocolate and vanilla thing. I was never a fan of them myself and pretty much always avoided them - didn't buy the dark shades - for that reason. But over the years I owned a lot of the lighter shades. The various shades of grey, and there are a great many shades, are the single most common color of toning there is. Even a lot of the coins folks refer to as "white", aren't really white, but simply one of the lighter shades of grey. That is because all coins begin to tone the moment after they are struck. And freshly dipped coins begin to tone just as fast as freshly struck coins, and some even faster, depending on conditions.
I enjoy some dark coins as long as they are not ugly to me. I think dark with nice luster or nice color are pretty, sometimes. Many would not care for this coin but I fell in love with it. NGC called it PF-64. I would not blame anyone that did not care for it.
If they are "deadly dull" with no luster coming up from below, I don't care for them. This 1913 Proof Dime is on the dark side. If you look at it under a strong light, the Proof surface shows, and there are no hairlines. Therefore it received a PR-66. Still it was not one of my better purchases.
I like to have wonderous variety in my collection. I collect Seated coins and have just about every version of toning you can think of. My guiding rule with a dark coin is that I can SEE the details of the coin. If I cannot grade the coin accurately than I pass. James
We are talking eye appeal here, and I like my coins as close to as they would have looked right out the door: bright, shiny, disks.
I like a nice original luster white silver coin. But the other end of the spectrum is also quite interesting.
A Peace Dollar which got away. I was outbid, by a fair margin too. I really crave that gun-metal color.