#1. "RE: blur background" In response to Reply # 0
Might help us to have a little more info of WHAT it is exactly you want, what shooting scenario you are dealing with ( portraits, close up of flowers..?? ) . Bokeh is a result of a a wide enough aperture to render your subject in focus yet your background out of focus.The quality of the bokeh is affected by the lens itself ( many are known for their 'creamy' bokeh, some not so well respected) and how far your subject is from the background. As far as using the flash , as far as I understand it , it doesn't have an effect on the bokeh. But maybe I do not understand your question
#2. "RE: blur background" In response to Reply # 0
Solano County, California, US
Silvio, If you are using flash, you need to be able to set a large aperture while making sure the flash doesn't overexpose your picture. It is much easier to get good bokeh when not using the on-camera flash. If you were using a speedlight or some other external flash, you could bounce the flash to reduce the amount of light hitting your subject.
What is the maximum aperture of the lens you are using?