Focus and exposure are 2 seperate issues, though longer exposures usually lead to blur pictures due to hand shake (if handheld).
Blur-ness can be due to shake of the camera, subject or simply out of focus. Subject movement cannot be controlled, but the other 2 can. Just make sure that you are using a fast enuff shutter (which may lead to underexposure if light or apperture is not enuff) for whatever focal length you are using.
Underexposure is due to too fast shutter speed or too small an apperture (large f numbers). Can be compensated by longer exposure and mounting camera on a tripod, increasing apperture size (stopping down the f number) or increasing ISO (leads to increased noise).
Oh, and one more thing, if the scene is too dark, the camera may not be able to see clearly and thus unable to focus properly, leading to your so-called blur pictures + underexposure...