blue smoke from exhaust on 55 reg vauxhall corsa?

why does exhaust emit blue smoke for about 5 mins from starting on my 55 reg vauxhall corsa diesel. I have used better quality diesel and added red x as well and although it has started to get better it has not stopped altogether. Diagnostics show no problems and car engine management symbol is not lit up on dashboard. it is regularly serviced and want to know if anyone can help please?