Blue smoke is burning engine oil, you usually get it cos oil is entering the cumbustion chamber when you throttle hard, (Worn rings or bore).... in your case though... the opposite is happening, when you take your foot off the throttle your getting blue smoke, thats cos when you decelerate, theres a big increase of vacuum in the inlet manifold, thats why you can use your engine as a brake,.....and somewhere in the inlet side, oil is being SUCKED into the inlet system then entering the engine to be burnt and emitted as blue smoke in the throttle off position. either oil is being sucked in at the turbo OR, oil is getting in at the inlet valve seals. the seals are cheap to buy but you will have to remove the camshaft and valve springs to get to them,