Digital BW, The Print

I'm told most of those online print on demand book printers use Indigo
presses. Apparently they are very hard to keep calibrated over even
short periods of time. This may not be much of a problem for economy
color printing, but a wandering CMYK printing process is going to have
a lot of trouble making neutral or just off neutral, or keeping it in
control when they do.
Complaints of green or magenta B&W from book to book have been common.
Doesn't mean it's impossible, just means you generally get what you
pay for...

Tyler