Digital BW, The Print

I'm trying to use QTRgui to print a TIFF from old VPSII * color
negative (early C41). 

QTRgui does OK directly from the color, but for some reason it doesn't
want to print from desaturated/adjusted B&W. 

What am I missing here?  

Black Only is handsome, but out of curiosity I'd like to use QTR,
which I was using a lot earlier today with B&W negative scans.


* Surprisingly this 25-yr-old 35mm color negative actually makes a
decent color print, scanned Nikon V with Vuescan. It was crossed over
when new and it's still that way, so B&W seems a good route :-)