BamBam is right. Male Blackbirds don't feed the females as such but will pull up worms for the female to take. This is his interpretation of courtship feeding. If he was pulling up a worm for himself and the female appeared, he'd leave it for her. Generally speaking, males don't feed females on the nest.
Maybe there is lots of food in your garden and the young are well fed and the worms you are seeing are worms the male is presenting to the female as a pair bonding thing and she too is well fed so they are leaving them though the urge to forage is still strong.
I hope someone as a better answer as i'm curious to know now :-)
That's interesting and makes sense, thanks both.