The trouble with blackbirds and thrushes is that they usually
do hide their nesting sites really well. And they're very discreet going in and out - nests are quite hard to find, as adults won't go near their nest if they think someone's watching them, so you can't follow them to it.
But, their chicks do tend to jump out before they can fly properly. Often there are two or three of them to a brood, and it gets pretty cramped. And if there's a predator (like, say, a cat) about, they do tend to 'explode' out of and away from the nest even earlier than they otherwise might.
Once they're out, they call for mum and dad to feed them quite loudly for another couple of weeks typically, they are quite noisy. They tend to stay away from open spaces, but if you can't fly properly, that doesn't help much. This all makes them quite vulnerable to cats, or lots of other ground predators (but mainly cats).
You can try to keep cats indoors when you think there might be young blackbirds fluttering about, but that's pretty much the entire period between April and August - thrushes start early and may have three brood attempts.
All that said, they're not doing too badly, considering.
https://app.bto.org/birdtrends/species.jsp?year=2019&s=blabi