Well,
Where do we start...
Piston Rings on their way out,
Breather system blocked somewhere,
Turbo seals ********,
Valve stem oil seals ,
Or the valve guides are worn.
Any of these can make for blue smoke as it is burnt oil.
When it does the smoke may help you as to whats the problem.
If you can give a few clues as to when it does it...
When left to tick over for a long time then rev'd
under full throtle moments
when you change gear when just pottering around
all the time???