I used Chinalco. Why is there a black spot on its surface? I want to ask if it's a fake.

You bought an OEM! No aluminum material in the market is directly named after Chinalco aluminum material. The real material is named after China Aluminum International Group Co., Ltd. Hengmei Aluminum or China Aluminum International Group Co., Ltd. Pingguo Aluminum. They are all branded, so-called fakes! Chinalco is the abbreviation.