It could be any of a number of things. Blue smoke is from engine oil getting into the combustion chamber.
Some usual suspects are:
Piston rings-
The nice thing about Detroits is that you can take the side covers off of the block and visually inspect them for wear.
Blower end seals-
The intake housing can be taken off to access the air inlet screen. You can remove the screen and look at the blower lobes to see if they are being "washed down" with oil.
Valve guides-
If the valve guides or stems or badly worn, oil can enter there. Although not all that common a problem, it is something to check, especially on an engine with lots of hours.
Air box drains-
Detroit Diesels are designed to leak oil out of these drains. That's the first place to look for unwanted oil entering the combustion chamber. If the drains are plugged or if it has an "oil catch" system to keep the floor of the engine room and the bilge from getting oiled down and it is clogged or full of oil, the air box can't drain and she will smoke. Some people that are not well informed about Detroits will sometimes plug these drains on purpose because they want everything to be "neat". Bad move!
Good luck with her.
