When I do greyscale comics, I honestly prefer to colour them in digitally!
I can't tell from your image -- what tools are you using to add grey? Are you thresholding your lineart so it's ONLY black and white & then using the bucketfill? Or have you got a multiply layer for the grey and then fill the area in with the pencil tool? There's a bunch of different ways to handle it, so if you tell us exactly what you're doing digitally, someone here might know an easier way!
But other options:
Option One:
if you're using Photoshop, you can use photoshop to remove the blue instead of scanning in black and white:
greyscale drawing on top of blue sketch, go to the "channels" tab
click on the "green" channel (starting in RGB works best for me), and after you've selected it, drag it into the trashcan and your drawing looks like this:
Now do the same thing with the "Cyan" tab and it should mostly get rid of your blue lines!
There might be a little ghost of your sketch left, but you can fiddle with Levels to get rid of that, and now you can convert your image to Greyscale and you've removed your Blue sketch while preserving greys. Of course, this only works if you can get a nice, consistent greyscale -- like if you're using markers or something. If you're just doing pencil shading it might look a little sloppy.
Option Two:
Don't worry about grey tones.
A black person doesn't need to be shaded in with dark grey to look like a black person. (Case in point: light-skinned or albino black people don't suddenly look like white people. They look like black people with super-light skin.) If there's no greytones anywhere else your comic it's a little jarring to just colour in this one guy!
