By "fix", I mean you're trying to use the phrase Default to mean your idea of default. I've never known (or maybe, don't recall) Default for printers to mean anything other than the printer vendor's default. If that's color, then color is what it will print regardless of what you actually want.
If I choose Default for our VersaLink C405, it goes to single sided color printing with "automatic" color. Which I never, ever use. I have profiles for the papers we use, which is why I have separate color options saved. Color management for the print driver is completely disabled, and the profile is chosen under color instead of just ColorSync > Automatic.
You're going to have to rethink "default" as something you have no control over. Those settings are set by the manufacturer in the drivers. Either that, or you can play with the driver itself and change it so the "default" is B&W.
If I double click the installed .gz driver, it extracts the text file with all of the driver information. I then did a search for default, which had 178 entries. I knew which one I wanted so I just skimmed through the text until I got to here:
*CloseGroup: PaperOutput
*OpenGroup: ImageOptions/Image Options
*OpenUI *XRColorCorrection/Color Correction: PickOne
*OrderDependency: 10 AnySetup *XRColorCorrection
*DefaultXRColorCorrection: Auto
*XRColorCorrection Auto/Xerox Automatic Color: ""
*XRColorCorrection Gray/Xerox Black and White: ""
*XRColorCorrection Commercial/Commercial: ""
*XRColorCorrection VividRGB/Vivid RGB: ""
*XRColorCorrection Vivid/Vivid Color: ""
*XRColorCorrection None/None: ""
*CloseUI: *XRColorCorrection
*OpenUI *XRColorAdjustmentSet/CMYK Color: PickOne
*OrderDependency: 10 AnySetup *XRColorAdjustmentSet
*DefaultXRColorAdjustmentSet: None
Now, if I wanted to, I should be able to change just the one line so Auto uses b&w, like this:
*CloseGroup: PaperOutput
*OpenGroup: ImageOptions/Image Options
*OpenUI *XRColorCorrection/Color Correction: PickOne
*OrderDependency: 10 AnySetup *XRColorCorrection
*DefaultXRColorCorrection: Auto
*XRColorCorrection Auto/Xerox Black and White: ""
*XRColorCorrection Gray/Xerox Black and White: ""
*XRColorCorrection Commercial/Commercial: ""
*XRColorCorrection VividRGB/Vivid RGB: ""
*XRColorCorrection Vivid/Vivid Color: ""
*XRColorCorrection None/None: ""
*CloseUI: *XRColorCorrection
*OpenUI *XRColorAdjustmentSet/CMYK Color: PickOne
*OrderDependency: 10 AnySetup *XRColorAdjustmentSet
*DefaultXRColorAdjustmentSet: None
Save the file, and make that the new driver. You don't have to compress it back to a .gz archive, it will work as is.
But if you want to try this type of thing, make sure you back up the installed driver first so you can put it back in case your changes don't work.