Hydrogen peroxide is H2O2. In the presence of water (just water), it breaks down into H2O and O (which then combines to O2). Which is why a lot of people put a drop into water they are trying to root cuttings in. In soil (or on plant tissue or skin) the molecule breaks down into OH, a bleaching agent. (This is where we get the term "bottle blondes".) The chemical reaction destroys cell structures, which is great for killing bacteria (and fungus gnats) but also great for killing plant and people cells. (This is one of the reasons that you should not use hydrogen peroxide on injuries.)
Honestly, I do not see blight or rot. To me, they look underwatered. I have a raised bed full of compost and I have to water a LOT because it is very free draining.