"black thick smoke" is not indicative of healthy motor and not a fact of life. The diesels do throw out a bit of soot under hard acceleration (not much) but not under normal driving. I can't comment on the reasons why it's suddenly stopped after an oil topup but I would definitely monitor the oil level after the next service. If it is using oil then you've got problems. Otherwise your smoke problem may be something as simple as a EGR valve, air sensor/s or injectors.