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it is not just the dirt from the road
ftfy. Foreign material contamination is definitely a thing, but in a clean room the wear particles are the culprit. I think the blackness of it is due to mixing aluminium oxide with transition metals, mostly iron but also Ni Cr V Mo Mn etc. which are present as alloying elements in the steel parts.
gbj_tester
snoops
interestingly, even chains on turbo bikes get black, suggesting it is not the dirt from the road…