Hi Steve,
No idea what you're blackbirds were up to, but I've also seen some very strange behaviour from birds that I've yet to find an explanation for.
This photo was taken a couple of years ago when I saw two blue tits fly around like a whirlwind, then lock on to each at about 20-30ft above the ground before plummeting to the floor, where they lay together like this for about a minute.
Then they got back up and started it all over again - about 3 or 4 times.
http://www.wildaboutbritain.co.uk/ga...ts&searchid=74
We'll have to try and dig out a bird behavioural psychologist (if there is such person) from one of the universities to see if they can shed some light on it.