A great photographer told me that the trick getting a pic thru glass when using a flash was to get up away from the lens of the camera and get the lens as possible to the glass as possible. The blur that you are talking about is caused by the camera moving (camera shake) or the subject moving while the shutter is open. When you are taking a picture in low light the shutter needs to stay open longer to allow the media to be exposed properly. A tripod is one way to reduce the movement. On some cameras you can set the camera to use a faster shutter speed. There are also some other ways reduce the camera shake... but I will leave that to the pros here
