Dark spot on back of cpu

[iquestkiller]

My P.C. wont post. After taking it apart to reseat the cpu I noticed a darkspot on the bottom of the CPU. I thought is was thermal paste but after wiping it with alcohol nothing happened. What is this. https://imgur.com/a/Szv573t
Specs
I5 7600k
Rx 470
Msi z270i bulldog
16 GB ddr4 adata 2400mhz

 

[hang-the-9]

I would contact Intel support and check with them. It seems a bit too odd to be just a simple color issue, maybe oxidation.