It's not yet clear if the printer is using the PCL or PS driver.
I just output a page here that has a 100% K block, a 20% K block, and a grayscale raster on my two color printers, an OKI C110 that uses a PCL driver, and my Xerox Phaser 7800 that is Postscript. As expected, the OKI outpts four colors, even when set to output all blacks accurately, and the xerox outputs only black, and registers a black click.