During the 1940s in Ireland the Flour used in baking bread came in bags made from linen, a couple of these bags sewn together made many a good sturdy pair of knickers.
Printed on the Flour bags was the firms name along with their motto,"the early bird gets the worm" and this bird was a Blackbird, we would have teased the girls by requests to see their"Blackbird".
Although I don`t remember any rhyme relating to this.