As long as only the blue line is driven ... you will have blue.  If you connect that line to the red and green pins you will have a black and white display.  You may want to connect the red and green ground pins to blue ground as well, but it's probably fine without.

I would suggest making up a small adapter to plug into the card's DA15 and connecting the lines across inside your adapter.  You could [1] cut a DA15 extension cable (male one end, female the other) in half and solder the wires together [2] buy one male and one female DA15 and solder a new set of wires between them, or [3] solder them down to a small piece of strip board and bridge across the R/G/B signal and RGB ground strips.