No brake fluid leakage is normal at the caliper. Either you were really sloppy bleeding the brakes, or you have a leak.

Balls of track rubber are not uncommon to see accumulated inside the wheels and various other places, but I'm not sure that's what you're talking about on your brakes. Track rubber isn't really tar-like. Just softish rubber from race tires that gets left on the track and picked up by your tires and then shed by them in little clumps.

If whoever assembled your brakes used a lot of grease on the moving parts (caliper pins, contact points of the pads, etc), that grease could have melted and run out when the brakes got hot. Without knowing exactly how much goo we're talking about, it's hard to say.