The term originated from the occasional black sheep which are born into a flock of white sheep due to a genetic process of recessive traits. Black wool was considered commercially undesirable because it could not be dyed. In 18th and 19th century England, the black color of the sheep was seen as the mark of the devil.
The transom is the flat back end of a boat. Some boats are pointed on both ends, so they have no transom. A traditional canoe, pointed on both ends has no transom. A modern square stern canoe (made to take an outboard motor) has a transom.
A pseudorandom variable is a variable which is created by a deterministic procedure (often a computer program or subroutine) which (generally) takes random bits as input. The pseudorandom string will typically be longer than the original random string, but less random (less entropic, in the information theory sense).