As I’m sure most of you know, birds undergo a moult of their feathers after breeding, which is why everything goes very quiet for a few weeks: they’re vulnerable to predation while their new feathers are growing in. This Blackbird appears to have completed its moult of body feathers but not yet its head. Either that or it has some kind of infection or parasite, perhaps, that’s caused this partial loss.
Fortunately, the bird’s baldy look is not affecting its ability to get about or feed. As I watched this morning, it cleared away a big pile of leaves and snaffled down the fat juicy worm it found.