When a girl has her period, a guy eats her out and holds the blood in his mouth, while the girl sucks the guys penis and holds the cum in her mouth. When their mouthes are full they makeout and mix the fluids.
In the enlish lands of enlang a person going down on a women on her period is right also in some of the US.
But, the most recent i found out is when a guy gets a few chicks together. Each wearing a different color lip stick. Each girl trys to get the lip stick the furthest down on the guys hard penis.