Magpies attacking blackbird and nest

A blackbird has a nest in some honeysuckle, very close to my back door. The eggs hatched on 29th June and the chicks are being well-fed by their mother BUT today there was an almighty commotion and when I looked out, a magpie was swooping down and chased the mother from the nest. There are a pair of magpies which I sometimes see on the roofs of houses opposite but I don't know where they nest. Is there anything that I can do to stop this from happening again? Is it likely that the magpie will try and take the chicks? The nest is not particularly well-protected, ie not deep in any foliage. Any advice would be very welcome!