Are they definitely rubbing because they are too tight? I've experienced blisters recently as a result of sandals being too loose and my feet moving around in them and hence causing the rubbing. I have one foot wider than the other and so nearly always have to choose between having one shoe too loose or one shoe too tight. So I usually but the size that fits the narrower foot best and then stretch the other shoe to fit but this only works if the uppers and lings are all leather.
If the uppers and linings are all leather then you might want to try my usal method which is just to just to initially wear them around the house with thicker socks on than usual! For examples if they're shoes you would wear wth tights or bare legs, wear a pair of cotton socks, if you'd be wearing them with socks anyway then wear two pairs! I've just kept adding socks until they've been gently stretched enough but you can see why I do this in the privacy of home! Clarke's also sell an aerosol stretching spray - you spray the shoes inside and out and then either wear them immediately or pack them with newspaper. I've not found that it makes all that much difference but it might be worth trying.
However, if the uppers or even just the linings are plastic then this won't work and, in fact, you might find in that case that there's little you can do about it because some synthetic linings just won't stretch and are actually designed that way so cheap leather uppers won't stretch too much and cause the shoes to become too loose in ordinary wear.
SM