That may be a layer of separation. Try to investigate without shaking it. Try to get a "core" sample. Stick a knife in there and core out a sample to see if it's darker in the middle or simply separation.
Oxidation would usually occur once it has been opened, with the introduction of oxygen. This could mean it has not been properly sealed and oxygen is entering. However there was a recent thread on this also, see if this helps::
https://thehotpepper.com/threads/sauce-darkens-aftdr-shipping.77037/