Darkening of top layer of jam

Hmm jam surface darkening ...........?
might be .
too much bubbles during processing and excess headspace during bottling..so as there is excess air it will contribute to "oxidative" browning .
Another reason not enough syrup to cover up the fruit bits so again it will lead to browning due oxygen exposure....

What is container capacity??

The first thought will be oxidation. Just another thought, it could be as a result of condensation which leads to dilution at the surface, higher water activity hence some microbial activity.

Darkening at top layer of jam can be happened due to false pouring process. Pouring should be done from less distance so less air will be entrapped and defects can be minimize.