Black and white has been a problem for me also, but not on black and white images inside the covers, which are consistent (I edit all, black and white, and color, in Photoshop). My frustration relates to a black and white cover.
A cover in black and white from a few years ago was true black and white, but the cover on my latest book arrived five months ago with a very unattractive greenish cast. When I have complained, Mary and Corey, et al, force me to send photos comparing the true black and white with the greenish black and white. This has happened four times for my latest book and the cover still hasn't been corrected. My insistence for supervision of their printer or for a new printer has been unheeded, and I am about to send a complaint to the Attorney General of my state for Blurb's repeated time-consuming demands, repeated delays, repeated substandard results.
Blurb is in the printing business, for heaven's sake!