Diesel engine oil does go black very quickly. Also a lot of garages suck the oil out to save draining it. This leaves a small amount of old oil in the sumpthat discolours the new oil. This shouldn't adversely effect the new oil due to dilution.
If it is dark brown rather than jet black, then I would suggest it has been changed.
Andy
If it is dark brown rather than jet black, then I would suggest it has been changed.
Andy