How would one correct for a blank in a potentiometric titration?
Even though the other compounds should theoretically not interfere with your titration, a method/matrix blank like you've proposed is always a good idea.  

What I would do is simply subtract the blank potential value from that of the sample for each volume addition, then plot and find the inflection point as you would usually do (i.e. from the derivative plot, etc.).  

If you are correct in your assumption that the other compounds won't interfere, then the position of the inflection point should be the same as that calculated without the blank, within experimental error (i.e. within a standard deviation or two or three, etc. of a set of replicate titration values.).  If you are incorrect and there is some interference, then the blank subtraction will correct for it and the blank corrected value is what should be used/reported.