You need continuous moisture for an extended period (like months) to grow black mold (the toxic kind). Plus, black mold typically doesn't grow on windowsills--it's usually found in places near leaking sinks or showers, for example. So you're probably not dealing with the really nasty stuff, that's the good news.
What you're most likely seeing is ordinary mold growing on dust, dander, dirt, mold, skin cells, and the like that have landed on the windowsills/edges. Mold doesn't feed on aluminum...but it does feed on debris from particulate in the air that lands on the sills. You tend to get mold problems right around the windows because of condensation from warm damp air being exposed to cooler temperatures on the window surface/sills.
First, you want to remove the existing mold from the surface. Ordinary household dish soap (mixed with water, preferably foamy) is adequate for this purpose. As a secondary effect, cleaning with soap will also help remove the debris that feeds mold growth.
Secondly, regular cleaning of the sills to prevent accumulation of organic debris will help prevent mold from re-growing. Reducing or removing condensation will also help prevent mold growth, such as using a de-humidifier, exhaust fans while cooking or showering, or opening a window periodically during dry weather to let moisture out of the home.