I found an answer to that, the black stuff
here. It says:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> Why does my foil darken or discolor sometimes during baking?
The darkening of foil may be caused by moisture from the food being covered. This results in a buildup of aluminum oxide. This is a totally harmless substance that is naturally present on the surface of foil. Moisture tends to accelerate this buildup.
Why does aluminum foil sometimes melt and leave black specks on the food?
Occasionally when aluminum foil comes in contact with a different metal or a food that is highly salted or acidic, small pinholes are formed in the foil. This is a harmless reaction that does not affect the safety of the food. It is difficult to predict, but may occur under the following conditions:
1. When aluminum and a dissimilar metal are in contact in the presence of moisture, an electrolytic reaction may occur causing a breakdown of the aluminum. To avoid this use aluminum, glass, ceramic, plastic or paper containers. Do not cover sterling silver, silverplate, stainless steel or iron utensils with aluminum foil.
2. A similar reaction may occur when salt, vinegar, highly acidic foods or highly spiced foods come in contact with aluminum foil. The result of these reactions is a harmless aluminum salt. Some aluminum salts are used in medicines to treat stomach disorders. The food can be safely eaten; however, the aluminum salt particles can be removed from the food to improve the appearance of the food.
</div></BLOCKQUOTE>