Do blueberries affect gelatin's ability to set?
No, the fault isn't with the blueberries. Certain fruits contain enzymes which denature ("digest" or separate) the amino acids in gelatin. Pineapple with the enzyme bromelain is one example. Blueberries don't have any protein digesting enzymes. The likely cause of your partially unset gelatin is not thoroughly draining the juice from the peaches. Possibly there was enough juice clinging to the peaches to thin out your gelatin. Next time, add a little less water when you make it.