Monk bowls are black because they are fired with sesame oil 5 times to protect it from rusting. There are two types of bowls allowed; Iron and Clay. Iron bowls need to be fired 5 times and clay bowls only need firing twice. Obviously, the clay bowls do not need to be black in color but are rare to find because they are fragile and very very heavy. Iron bowls are black from the baked-on sesame oil. The oil is also treated or burnt beforehand too.
Today, we have stainless steel bowls, and since steel is made from iron, it also must be fired 5 times. We must count it as iron because aluminium, which is also part of the steel alloy is not allowed for bowls (by itself) because it is too weak. Stainless steel does not usually rust, but it still can rust. Ask anyone who has stainless steel knives. Some traditions like the Thai tradition rarely fire their stainless steel bowls because they say there is no purpose or need to protect it from rusting. Some traditions in Thailand fire their bowls only once or twice to make it look ugly so that others will not steal the steel bowl. No bowl, no eat. Some of the cheaper bowls in Myanmar are painted to give it such a look. Not only is this bad for health, such bowls do not last very long.
Above: A newly baked bowl. Below are pictures of monks firing their bowls to protect it. The first pictures are done at Pa-Auk with a gas fire and another is done with a wood fire at Varanasi Monastery in Mingaladon, Yangon. A metal plate goes above a fire (gas or wood), and an upside down pot with a handle on top serves to help bake the oil onto the bowl. When it is near to being finished, the blacksmith monk will take some ash and sprinkle it onto the bowl and then he will blow it off. If the ash sticks to the bowl, it needs more cooking time. If it blows off clean, then one round of firing is finished. Usually the monks will do several bowls in the same sitting to conserve fuel.