If you desaturate them and then they look blue, then I think you need to calibrate your monitor.
If you already shot them on BW400CN film, then simply desaturating should be fine. If you wanted to convert color images to B&W, desaturating will most often give you a flat look.
If you have a digital camera and want to try a free color ->B&W converter that mimics using different colored filters with B&W film, try the B+WPlus filter from cybia. (link
here).
It's freeware, as are many of their plugin filters. But of course you are shooting film, and black and white at that, so for these plugins you'd need a color digital image to start.