The presence of some white feathers is usually called a leucism, a phenotype resulting that means a reduced pigmentation (in birds, feathers usually turns pallid or white). But this case isn't produced by high frequencies of genus that order leucism feathers, it's produced by the 'progressive greying'. Just like us, older birds show more grey/white feathers caused by the age. You should read that article, where is better explained.
'Generally with blackbirds, their breeding success increases with age when they are more successful at nesting.
'A bird with white feathers would suggest to the opposite sex that they are experienced breeders.'
(taken out from the article mentioned above)
I recaptured that female Blackbird last Saturday. The bird was first-trapped in May 2012, already with white feathers. I managed to take some more photos.