Pre-1982 cents are copper alloy. They can exhibit a similar effect as your coin when that alloy is improperly mixed. You can see a bunch of them at this thread here. Cents minted after 1982 are made of zinc with a copper plating. Your coin exhibits improper plating. If you like it, keep it, collect it, and enjoy it!
The 1970 and the 1968 are not improperly mixed alloys. They are just dark because of Environmental Damage and toning.
I see you used this term more correctly than most here do. I've often wondered why some or most say copper when they are actually bronze. Are we confused or are we confusing others or maybe it really does not matter? Just wondering.
