This is not scientific research, just what I can make out from my local blackbirds
A soft chook-chook-chook: parents use this when they are looking for food. It seems like they are trying to ascertain where each other is and if they've found anything.
Chook-chook-CHOO-COOK-COOK-COOK: alarm rattle. Sparrowhawks, cats, seagulls, crows, big flocks of anything. They do a softer version of this which I feel means happiness, or an expression of success - thus a beakful of food is often accompanied by a chook-chook...etc as they fly into the nest.
Seeep. Seeep. Seeep: keeping in touch with the chicks, I think.
Our late blackbird's dusk song partially consisted of variations on Pop Goes The
Weasel; our new bird is a bit erratic and hasn't really got a theme song
The only time I've had a blackbird come as close as yours was when Mrs Beaky was feeding chicks. Because I spent weeks defending the nest from the local moggies (and had many a surreptitious peek inside) I think she thought I was safe enough
