A blu ray player is no different from a regular dvd player in that regard. Whether or not a movie fills the screen, on either a dvd player or a blu ray player, depends on the aspect ratio that the movie was filmed in originally.
Most theatrical movies are filmed in a ratio which will result in black bars on the top and bottom on blu ray, just like dvd.
The only difference is in the quality of the pic, dvd=480, blu ray=1080. Not the size of the pic.