Ditto......
"Carbon Black"/ soot is generated by all diesels. It is in every oil galley, cooler and passage in the engine. Oil designed for diesels has heavy detergents and other agents to pick up the carbon and keep it in suspension so it does not cake up and create a blockage that would cause parts of the engine to be "oil starved".....I would worry if the oil did not turn black!
Cheers,

__________________
FRED

Daily Driver: 98 E300TD 199K
Hobby Car: 69 Austin Mini
Past Diesels: 84 300SD, 312K
87 300SDL, 251K
94 Chev. K-1500 6.5Ltr.TD, 373K