Blackbird sitting on Robin chicks!

We have a Robins nest in the climber plant at the side of our house. It's beside our seating area, so we've been watching the Robin parents tend to the chicks for the past week or so. 

This morning I noticed a blackbird perched looking at the nest. Then it suddenly flew into the Bush where the nest is. 

I popped out to have a look & the blackbird was sat in the nest, on top of the Robin chicks. 

I just wondered whether this was typical behaviour? And whether blackbirds commonly take over other birds nests?