of course, utnik is right.
but if you're printing on laser printers, you don't even need the hassle of converting the png into tiff.
just use the same colors in the png and in the scribus file.
and since png supports transparencies, just make the background transparent.
from my experience, the only reason for using matching CMYK colors in both the images and scribus file is when your sending the files for profesionnal printing and the print shop requires a pdf version that does not support transparenices (or you don't want to take the risk that the print shop incorrectly flattens your pdf)
otherwise, use the same rgb blacks and/or use transpancies...
for a black file with png cliparts i wouldn't even bother to use any color management...