The oil is becoming diluted with diesel. Drain it out, and replace with fresh uncontaminated oil. For blue smoke to form, the oil level must be sufficiently high that some of the oil is being forced into the cylinders. This exchange of oil and fuel isn't good! I don't know why the contamination is occuring-perhaps due to an active DPF regeneration which requires diesel to be sprayed into the exhaust gases. Some of this might be running down the cylinder bores into the sump.