Oh, my god, I did write "bleet." That's too funny. Yes, purely a typo, as the book clearly has it as "bleer." By the way, the Canongate edition gives a few translations of some of the dialect in the song:

simmer = summer
heugh = runs, cliff, or crag
waukin = waking
wauk = wake, restless
nane = none
lanely = lonely
lave = rest/remainder
bleer = blur