Turning brown on top is usually a sign of too much oxygen in the jar.
The NCHFP FAQ covers this exact topic (search for "dark"). There are a few primary causes:
- too much headspace, or bubbles left in the jam before processing;
- not enough liquid to cover bits of food/fruit; or
- not enough processing time.
All of these result in the surface of the jam being exposed to oxygen. The first two are fairly direct: there's too much air on top. The third is because processing drives some of the air out of the headspace, leaving lower pressure air, which not only prevents browning but also creates a reliable vacuum seal.
Too much headspace is an easy possibility for you to check. The right amount varies depending on the jar size, but you might well have too much.
The processing time is the really worrying thing to me in your case; you're not processing your jam at all, just packing it in hot and letting it seal. That's actually a good way to get unreliable seals, but it sounds like you're getting lucky and it's working out. (But be really careful to test the seal later just to make sure after it's cooled - push down on the middle and make sure it doesn't pop back up.) Still, I might suggest trying a couple batches of jam with a tested recipe that involves water bath processing just to be sure. It might well solve your problem, and it'll make your canning safer.