A raster image is like an image made up of tiny floor tiles. When you try to "enlarge" it, all you are doing is spacing the tiles farther apart from each other and the image software uses an interpolation method to "fill in" the spaces with additional tiles. Or another way to think of it, is like when you imprint a newspaper image on to Silly Putty, then stretch it. In either case, you're not "adding" any detail by enlarging the image, hence the blurriness/softness.
Those plugins for Photoshop which claim to help you enlarge images without them looking blurry, basically just use algorithms to detect edges in the photo and keep them sharp, giving the appearance that the overall image is sharper. It helps a little, but again, you're not adding any detail to the image.
Your best bet is to just try and obtain a higher resolution copy of the image.