Most semi-autos get that from ejecting brass - Hoppes will usually take them off.
Be aware that if you shoot aluminum cased ammo - there is no way to get these off. I prev had them on my USPc slide from the aluminum cased ammo. The stuff that takes off the brass marks won't work on the aluminum marks.
Everytime I shoot the gun, however, you will just get more.
The slide on my Beretta PX4 is inset a great deal around the extractor area. So, it doesn't get these marks.
Also, with my slide being hard chromed, I no longer get these marks on my P99, P99c and USPc.