If it looks a bit scruffy, it might be that it has just got its adult plumage. I have one in my garden - some of its feathers are sticking out at odd angles, making them look light coloured rather than black, giving it a look similar to that which you mention.
It's quite common Arthur for Blackbirds to have white flashes like this. We have had several in the garden over the last 25 years. I presume its rather like us having a streak of grey hair (my son has had one since he first grew any hair!) I am sure someone more knowledgeable than I can give an accurate scientific reason for this.
Arthur. I have found the following link which might interest you.
Elfin, my Partridge has gone. She hung about watching whilst they ploughed the field behind as as soon as it was done she took off. I only hope that come October the guns don't get her and that next spring she finds a new mate!
Arthur, I forgot to post the link after all that. Forgive me for changing the subject but Elfin and I had had some discussion about a pet partridge of mine some time ago and I thought I would let her know what had happened.
http://www.garden-birds.co.uk/birds/blackbird.htm
Arthur - are you in UK?
If so, do you mean a black bird or a blackbird...?
If the former, could be a jackdaw, rook or crow, especially a young one -- they often show the white bases to their flight feathers. If the latter (thrush-like, hops, yellow bill & eye-ring) yes, they can have white patches and flashes, often but not always asymmetrical .