After you have cleared it up, keep cleaning it with the bleach solution that you are using to prevent it from coming back. You're just going to have to clean it as a regular part of bathroom cleaning since mold is obviously a problem.

If your windows are usually covered with shades, that can be a contributing factor because it traps the air. Also, houses are built so air tight nowadays that condensation on windows is an issue for many -- especially when the indoor and outdoor temperature varies greatly. Be sure to keep the humidity in your house below 36%. You may have to measure this because what feels like low humidity to people and what the humidity actually is, can be two different things. Showers, cooking, etc., without ventilation to the outdoors can increase humidity in your house.