Why is Black Gate the Gate of Hell?

Folks describe "Black Gate" as "black evil spirit".

In some places in the northeast, the black door of the house is called the "black door". Although it is black, it is not painted, but it looks like a colorful door god painting-a symbol of "darkness". Folks describe the "black gate" as "black evil". It is said that "black evil" is the gate, and evil spirits are hard to invade. The color of the door became the door god.