Dark spot on back of cpu

Jan 16, 2019
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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