Black dirty spots on front rim

I just bought a used 2008 triumph Daytona 675 with 12,400miles. I took it home and noticed black spots on my front rim. I figured it might have been from the road so I cleaned the rim. After a second ride the spots were there. What could this be from? I am assuming it has to do with the brakes. But not sure what part. I wrapped a zip tie to my front brake lever and throttle squeezing it. Going to leave it there overnight to see if any leaks from the brakes. I would hate to clean the rim after every ride.