blackening

ˈblak(ə)n

verb

make or become black or dark, especially as a result of burning, decay, or bruising.

stone blackened by the soot of ages

make black,black,darken,make dark/darker,dirty,make dirty,make sooty,make smoky,stain,grime,begrime,befoul,soil,grow/become black,dim,grow dim,cloud over