Black Snakes is a type of firework made with sodium bicarbonate and sugar. The result of these chemicals mixed with fire produces a carbon dioxide reaction, the result is a thick black coil of ash resembling a snake.
In the episode "Summer Sucks", all fireworks were banned in Colorado, except for black snakes as a result of a boy from North Park blowing his hands off with one. No one in the town of South Park was interested in or excited by black snakes, but the town Mayor wanted to have a big, exciting 4th of July fireworks show.
The mayor enlisted Charlie, a pitchman for the Dyno Might Firework Company, to create "the world's largest black snake". It was significantly more dangerous than any of the other fireworks, but because it was still technically a snake, it was legal. The giant snake would go on to cause massive amounts of damage around the world, however, and later was put an end to by a bunch of fireworks, which caused it to snow black ashes. The townspeople actually enjoyed this, but when Chef got back, he thought it was a racist joke and asked them to get in a line so he could "whoop their asses".