Do you want to build a Drive Master with a blend from scratch or are you trying to modify an existing DM? I don't think I would mess with modifying one to blend because you're going to have to find some where to put an op-amp on there and you're also gonna need to get power to it. If you're building from scratch I would just split the input right after that 3k3 resistor. Get a dual IC and run the clean signal through a negative feedback loop in the IC. Then combine the output of the drive master circuit and the clean signal with a 50k potentiometer (your new blend knob). Take the new output and run it through another negative feedback loop on the other side of your dual IC. The output from that will now be the output of the pedal. Don't forget to put an output cap on there too. Good luck and btw, I'm no expert on this stuff so someone else may chime in and explain why my approach wouldn't work at all. Good luck and try researching DIYstompboxes.com for a better answer.
MA's answer was pretty comprehensive. If you understood it then you'll know why the mod may be difficult; if you didn't then read up on diystompboxes as he suggested.
