Not having much luck so far, without further information to go on. Rhiannon, who is Chair of the Welsh Folk Song Soc, tells me that Tegwyn Jones' book, "Ar Dafod Gwerin" has:

Un frân ddu, anlwc i mi,
Dwy frân ddu, lwc i mi,
Tair brân ddu, cariad i mi,
Pedair brân ddu, priodas i mi.

that is ...

One black crow, bad luck for me,
Two black crows, luck for me,
Three black crows, love for me,
Four black crows, wedding for me.

These sorts of verses are 'folk' poetry - largely unattributable - sort of in the same vein as "Red sun at night ..." in English. I don't know where my copy of Ar Dafod Gwerin is right now so I can't check if there are more verses.

Again - it would help to know where you've heard the one you're looking for.

sian