How are you lighting your subject and the blue screen? For chromakeying you need to light your subject and the blue screen as evenly as possible, if not small black particles will appear. It also depends on what software you are using to chromakey and the camcorder. A camcorder with one CCD or CMOS will never give you the results of a 3 CCD or CMOS camcorder. The basic three point lightning apply here, so the only solution is trial and error until you get close to a perfect result.
