The darkening is oxidation. It's normal.
Both ETP copper and oxygen free copper will oxidize exactly the same in the presence of air. The term oxygen free copper does not mean that it cannot oxidize. See this Wikipedia link:
https://en.wikipedia.org/wiki/Oxygen-free_copper#Use_in_home_audio
"The high-end speaker wire industry markets oxygen-free copper as having enhanced conductivity or other electrical properties that are supposedly advantageous to audio signal transmission. In fact, conductivity specifications for common C11000 (ETP) and higher-cost C10200 Oxygen-Free (OF) coppers are identical; and even the much more expensive C10100 has only a one percent higher conductivity—insignificant in audio applications. OFC is nevertheless sold for both audio and video signals in audio playback systems and home cinema."
"High electrical conductivity coppers are distinct from coppers deoxidized by the addition of phosphorus in the smelting process. Oxygen-free phosphorus-containing copper (CuOFP) is typically used for structural and thermal applications where the copper material will be subject to temperatures high enough to cause hydrogen embrittlement or more exactly, steam embrittlement. Examples include welding/brazing rods and heat exchanger tubing."
"Copper alloys which contain oxygen as an impurity (in the form of residual oxides present in the metal matrix) can be embrittled if exposed to hot hydrogen. Hydrogen diffuses through the copper and reacts with inclusions of Cu2O, forming H2O (water),which then forms pressurized water steam bubbles at the grain boundaries. This process can cause the grains to be forced away from each other, and is known as steam embrittlement (because steam is produced, not because exposure to steam causes the problem)."
