Not all black molds are the "black mold" known scientifically as "Stachybotrys" but in all cases of mold, extreme care should be taken so that it doesn't become airborne and ingested.
If you are concerned about the type of mold, I would recommend you contact a testing agency within your area to confirm the type of mold and health risks associated with it.
In your case since it is on alumninum it should clean up easily due to the non-porous nature of aluminum. More than likely this may just be mildew from condensation and air infiltration.
Mold and mildew develops on your window sills because the window frames are not thermally broken.This allows condensation to form on the inside of the frames during cold days and winter months.
- The formation of condensation could be an indicator that your humidity level inside the home is too high.
- To check this you can purchase at your local hardware store or home center a countertop humidity gage.
If you have the capability, try to adjust the humidifier to maintain a level below 40%.Maintaining this level should reduce the condensation and mold/mildew build up on the frames and also reduce the growth of dust mites.
For more information:
Read "Our aluminum-frame windows are harboring condensation and mold. Can you recommend window solutions for a damp environment?" a Q&A answered by Steve Saunders.