The stain is an indication of a fire going on inside the cylinder.
Racing oils normally have slightly less cleaning dopes than other oils, so a slight carbon build up will not be cleaned again. This is normal.
When you look at the inside of the piston you will see a brown discoloration. If that turns to black it is a sign of very high operating temperatures. If it turns greyish you are on the verge of burning a piston (death ash). In that case, the black on top of the piston also has turned grey.