Ohhhh yes, someone leaned against the bike or let a strap hang down.
Go for a quick 5-minute ride, put a piece of cardboard behind the pipes to prevent overspray, and hit it with a light coat of this fume-free oven cleaner. Grab some wet paper towels layered up thick, so you don't burn your hand, and it should wipe right off.
Don't get it on your paint, or you'll be a sad panda with faded paint spots. If it drips on chrome, doesn't matter.
If the black mark doesn't wipe off immediately, have more damp paper towels, spray the oven cleaner directly on the towels if you like, and wipe wipe wipe. Don't scratch, no firmer than a good rub. Smooth motion in one direction if you can.
First couple of times I had to clean off various females high-heels melted onto my pipes, this did the trick over and over, so great. Then I put passenger floorboards on, and my now-girlfriend already knew to keep her feet away from the hot melty stuff.