After mating, she usually kills and eats her mate. She's black and now has no 'spouse' so she's a black widow. But it is not any more or less usual for the black widow to eat her mate than it is for...
They get the hourglass because if they bite you, you mite die in an hour.The original answer has been left because it is amusing. There is no real reason or benefit for the coloring of the black...