Blackbirds make a pretty sturdy high-walled nest so it wouldn't have fallen out. I guess the chick would already be fairly big because if small and die in the nest, the parent drops it somewhere well away from the nest for obvious reasons.
The chicks leave the nest when they are only in a half feathered state and in towns often die of cold or/and hunger. Intense competition for breeding territories and unsuitable weather can put a pressure on the food supply (worms/bugs etc. so the weakest chick(s) will succumb.
Sad, but Nature dictates that it is better to have 2 healthy chicks than 3 unhealthy ones that probably won't be able to breed next season (if they make it).