This is David Snow's description of the blackbird's courtship display:
The fully developed courtship behaviour of the male is the blackbird's most striking display. The head, with the crown feathers partially erected and the beak open, is stretched forward; the neck-feathers are compressed, and the body feathers fluffed out, especially the feathers of the rump, which form a conspicuous hump; and the tail is fanned and depressed. The displying bird has a curiously wild, staring appearence.[...] The whole time, with his beak held open, he usually utters a low 'strangled' song, made up of chattering alarm notes, rough warbles and subdued snatches of what sounds like true song. If the display is performed in a tree, the male remains stationary or at most occasionally shifts to another perch near by, while the bowing part of the display is more prominent, developing into a rhythmic up and down movement of the whole head and neck.This display takes its full form during pair formation, unpaired males - young males or widowers - court females most often in February-March. Paired females attack displaying males, unpaired females appeared indifferent, although they might later mate with the male.
A brief form of the display occurs just after females solicit copulation in the days prior egg laying. According to David Snow, copulation is always interfered with by other males, who jump at the copulating male, often knocking him off the female.
This is the first photo I managed of the blackbird singing
Snow, D.W. 1958. A study of blackbirds. George Allen and Unwin Ltd.