kpyip, since you have that Night Portrait function, I would recommend that you use this as a learning tool before going manual setting.
Night Portrait function usually use higher ISO to capture background lights while built-in flash helps to 'light-up' your object. At this function, expect the shutter speed to be slowed down & you might need a monopod or tripod or IS on your lens.
If you have a blur object than the background, then your focusing could be on your background instead of on your object. If the blur is caused by camera movement then object & background would both be blur. Hope this helps.