Actually it is VERY normal. What you're seeing is aluminium powder from the brake pads wearing out the rims.
It's impossible to avoid some dirt and grit getting on the rims and when the brake pads squeeze the rims the grit transfers to the pads and embeds itself into the softer pads. Now you've just turned your brake pads into a lapping block and the rims are the workpiece being lapped down. This really accelerates when you ride in wet conditions and the result is the black/dark grey goo that you're seeing all over the back end of the bike.
The situation seems to be worse for the rear wheel. When riding in the rain I've had lots of time to think about it and I've come to realize that the front sees a lot less of this grit, but it's far from immune to it, but the front picks it up and throws it at the rear wheel. So the rear is always about 3 times more dirty with this stuff than the front.
Commuters are aware that wheel rims are a consumable every bit as much as tires and chains if you ride long enough. If there's rain involved and you're riding a few thousand kms a year you're lucky to get more than two years out of a set of rims. Mountain bikers that ride in muddy conditions know this all too well if they are still using rim brakes. It's not uncommon if you ride trails a lot to go through a set of rims in a year or less.
One way to help is not to use sandpaper when resurfaceing your pads. Instead get a coarse file and use that. Sandpaper breaks down in use and when that happens the grit embeds itself in the faces of the pads you sand. Switching to a metal file will avoid this issue. But this is a fairly small point since as soon as you roll the bike out on the very next trip the process starts all over.