Some blow-by is normal on any diesel engine. This is because the combustion pressure is just too great for the piston rings to hold completely.
Excessive blow-by can caused by piston rings that are sticking in the bore. One of the ways to check if blow-by is too great is to put your oil filler cap upside down on the filler hole. If the cap gets blown off, there is too much pressure in the crank case.
Blow-by can be caused from piston rings that are worn out. A cylinder with worn out rings will have low compression and will likely cause a misfire. These piston rings need to be replaced.