A reasonable approximation for the primary optical effects would be a gaussian blur and it will produce a visual effect like "dark bleeding into light" just as well as "light bleeding into dark". Total energy is conserved under the blurring function, so light spreading out to the neighborhood leaves less light to perceive as being at its true source location. So, the light source seems darker at the same time it makes its neighbors seem brighter. Without uniform reinforcement from its all of its neighbors, the light area will seem to dim.
I think that several other comment threads get closer to the truth of the matter. Your experience will be quite different depending on the dilation of your pupils. Most people have better acuity in bright environments where their eyes behave more like pinhole cameras. Not only does the pinhole have better depth of field than a larger opening, but most people will have more higher order aberrations in the periphery of their cornea. That is why they will see artifacts at night more than in broad daylight.
With a properly balanced viewing environment, you will have the same acuity for light on dark or dark on light, because the ambient lighting should dominate your field of view and set your pupil dilation. The screen should then be bright enough to fit comfortably in the same dynamic range as this environment.
Another factor is screen surfaces, and what people perceive in the dark/black areas. If you use glossy screens, you are more likely to notice distracting and non-uniform reflections in the darker areas. With a matte screen, you will mostly just have reduced dynamic range with the environment affecting the whole screen as a more uniform noise floor. Smudges on touch screens may also affect someone's perception of solid colored screen areas.