"Blackbirds" are a very specific bird in UK English, Turdus merula; in Irish londubh (sometimes ceirseach for the female).

There's a Wiki about this rhyme.

As for the four-and-twenty, how numbers are expressed varies from time to time and place to place; in Irish forty is expressed as dá scór (two score) in certain contexts; in French eighty is quatre vignt (four twenties).