Answer:
Because they coat asphalt (Bitumen) into their boats.
Step-by-step explanation:
In the Mesopotamian text, Magan was always mentioned as a civilization with one of the most advanced maritime technology in the region.
Coating Asphalt to a certain object will make that object become more waterproof.
It is believed that the Magans aware about this and decided to coat their boats with Asphalt to reduce the chance of sinking. The asphalt coating is the one that resulted in the black color of the boats. This is why they were called the “Black Boats of Magan”