If it can be wiped off I wouldn't really call that a blemish. If I clean my P365 with 99% isopropyl alcohol it lightens to a dark, uniform, almost grey color. But with any kind of oil, gun oil, hand oil, etc., the surface becomes much darker. This is normal. But if applying gun oil to the slide and wiping off all of the excess doesn't produce a uniform looking slide without blemishes, then you might have a cosmetic issue to talk about with Sig.I took it to the shop the guy that sold it said the blemish can be wiped off.