Although symbols have a host of meanings and nuances in various cultures, blackbirds are historically associated with evil in Western culture due to their color. In Christian tradition, they were recorded in the 12th century bestiary to be symbols of being tempted with carnal desire.
Blackbirds, like most birds, also symbolize freedom and the link between the the temporal and the eternal in many cultures. In a New Age context, they tend to symbolize secrets and mystery, and have lunar connotations. The blackbird, being a highly intelligent animal, can also symbolize the human soul, specifically human intelligence as well as wisdom.