Black/dark grey copper coin? Hamm 1739 3 pfennig [solved]

Recently I bought a 1739 3 pfennig coin from Hamm city: N#9462

Its colour is dark grey, which is odd for a copper coin. The weight is 0.3 grams less compared to the given value, but that seems like a possible weight variation for a coin of that age. The size seems fine as well. It is a relatively low value coin, it does not make sense to make forgeries.
Some of my other copper coins are very dark brown, but none dark grey like this one.

It seems to me that this coin has possibly altered chemically slowly, obtaining a layer of dark grey CuO: https://en.wikipedia.org/wiki/Tenorite

Does that make sense? Under which conditions do copper coins obtain such a dark grey colour?
Does anyone else have any old dark grey copper coins(s)?
 

Left: pictures of the coin made with direct sun light. Right: the coin next to a normally circulated regular copper coin for colour comparison.

Being in the ground and luckily not being effected/damaged by moisture or soil acidity for a few hundred years could explain the darkness. I know I have dug up old coins which have been darkened by being buried, all depends on the conditions though…

Also, I see you are from Nederland - you crazy Dutch buy lots of ground earth from building sites all over Europe to reclaim the parts of your beautiful country which are below sea level, I assume it could have been found from this reclaimed top soil. Pure speculation on my part, but could be one explanation.

Thank you King, this is exactly the kind of answer I was hoping for! I suspected it to be a coin that had been in dry soil for a long time, your experience with digging up darkened coins sure helps to consider this most likely to be the case indeed. Assuming this is what happened to the coin indeed, it still is quite amazing to me it retained such a level of detail. A previous class mate of mine at that time locally found some coins with a metal detector in an area with forest and sand dunes, the ones he found were of much later date and in a terrible condition.

And yup, proud to be infected with the Dutch mad way of thinking. 😉