could be a number of things
1) Oil rings on pistons; but a comp test will not always show this as being the cause
2) guides or stem seals
3) turbo
rule out the things you can do yourself, so go and find a big hill and let the car overrun until you reach the bottom, at the bottom give it a boot and if you get lots of blue smoke then i suspect stem seals or guides
you can also do a comp test, but as said it, it will not always be conclusive. A cosworth YB i rebuilt years ago had the same issue, a comp test wasnt conclusive, but once stripped down it was clear to see it was the oil rings.
you could always whip the turbo off for inspection; you will normally find a wetness if its oil seals