I live in Battersea, with a garden with plenty of trees (too small for nesting). Last year was a bumper year for blackbirds. I was on the edge of two territories so there was a lot of competitive singing and quite a few face-to-face standoffs. In the hottest weather one bird used to sunbathe; they fed on worms, and on flying ants in season; they parked a fledgling for its first couple of days outside the nest.
This year – nothing. No songs audible anywhere, even in the distance. No sightings either.
It is not just Battersea. I have friends who live in Chelsea who have seen and heard nothing from the blackbirds this year. And in Wandsworth.
Is there some sort of population crash I haven't heard about?
I can only comment on my own urban back garden observations, and here the blackbirds have been quiet this year after likewise, a bumper year last year. I could be due to the weather being wetter and food being more abundant elsewhere, though I doubt that too much here, because we had feeders up and ground feeders for ground feeding birds, currently all removed due to a rat concern in the last few days.
I'm tempted to think they may be travelling before settling down, but only a wild guess, and I'm sure someone will correct me if that is totally wrong.
I'm not overly alarmed because there was a lot of youngsters and we seem to be getting an influx of blackbirds returning as the weather turns colder.
I think it may be due to a poor breeding season. If you recall we had a cold spring this year. There were a lot of reports of Blue and Great Tits dying in their nest boxes because their parents couldn't get enough natural food or the cold. I think that many Blackbirds may have suffered a similar fate. I certainly didn't see many young Blackbirds in the garden. By contrast spring 2020 was warm and sunny and most birds had a good breeding season. That's my perception although there may have been other factors at work as well which I'm not aware of.