I'm sure there are BETTER ways to do this - those on PJ who are good with channels probably know something. But since I'm self-trained in Photoshop, I usually just go with the first thing that works for me.
ONE way is to:
1. Go to Image > Adjustments > Hue/Saturation.
2. In the Hue/Saturation pop-up window, there is a drop-down menu at the top left that says "Edit." The default choice is "Master"; change this to "Cyans."
3.Move the "Lightness" slider all the way to the right (+100). This should turn your blue marks white, thus melting them into the background.
4. If you still see some traces of blue, change the "Edit" drop-down choice to "Blues" and repeat step 3.
5. If your blue pencil is a really funky color, you may have to play with the other "Edit" colors and try step 3 again until all traces of blue pencil are gone.
This should leave you with a clean black and white image. NOTE: If you use blue-line paper for your art, this method will remove them too. If you just want to remove the blue PENCIL and not the blue LINES, make sure to select the area to be corrected with the select tool first.