I am bringing my ArchiCAD drawings into plot maker and I tell plot maker that i want to change all of the colors to black when they print but they are still comming out gray unless the color it is drawn in is actually black or white.
I remember having that same problem when I first started with Archicad. I had a problems printing because I was trying to "Plot" the files. I don't know if that's what your doing or not, but a lot of my problems stopped when I started to "Print" to the plotter. You may already be printing instead of plotting, that's all I can think of.
Where you are setting that the drawing should be black&white ? In the plot dialog box or in Drawing properties? Did you setup your plotter properly in AC ? Eventually you can try to change the pen colors to black in "All Pens and Colors" dialog box.
In my opinion it is better to control your output by defining the output color in the Pens and color dialog box versus choosing the all colors to black option. You might explore this when you have a chance. Archicad 10 allows multiple pre-defined pen sets to be used and assigned to particular drawings for output