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.
trans-Across, through, over, beyond, to or on the other side of, outside of. A compound in which two atoms or groups are situated on opposite sides of some plane of symmetry passing through the compound. (Also used without the hyphen as an adjective; see trans.) Transgender or transsexual, or pertaining to those who are transgender or transsexual.