I think it is the other way with gasoline engines being blueish white for OIL - black smoke for gasoline...
Just checked google search about the different colors coming from the exhaust...
FOR GASOLINE ENGINES
Black exhaust smoke is an indication of a rich fuel condition.
BLUE/GRAY exhaust smoke is an indication of oil burning in the combustion chamber.
White exhaust smoke is an indication that coolant is burning in the combustion chamber.
FOR DIESEL ENGINES
Black smoke is the most common emitted from diesel engines and indicates incomplete combustion of the fuel.
Blue smoke is caused by engine lubricating oil burning.
White smoke is caused by raw, un-burnt fuel passing into the exhaust stream
At least that is what a couple of GOOGLE SEARCHes says...

Roy Ken