H2O2 (hydrogen peroxide) has the same atoms as water, the difference being the extra O- (O negative) free oxygen radical it contains. The free oxygen radical makes H2O2 a powerful oxidizer (more so than household bleach) that attacks any organic molecule it contacts. It is responsible for the destruction of chlorophyll that occurs when leaves are unable to tolerate the light intensity they are exposed to. As the overexcited photosynthesizing machinery returns to its normal state, the free radicals of oxygen are released and immediately attack the first organic molecule they contact, which is usually chlorophyll. The damage caused is called photo-oxidation but more commonly called sunburn.
No matter how you use it on living tissues, the good will always need to be weighed against the bad, and there won't be a question of whether injury occurs or not, it will be how significant the injury is. There are many products used on plants that are always phytotoxic and cause injury, dishsoap, for one. Another example, acetic acid (vinegar), can be used to acidify irrigation water w/o issue, or serve as a nonselective herbicide at more concentrated solutions. So the poison is in the dosage.
Blights are a best avoided by prophylactic applications of fungicides. They are caused by a long list of pathogens (bacterial, fungal, oomycetes, all of which commonly occupy intracellular spaces (spaces between cells within the plant) or are active in xylem tissues or the plant's vasculature. As such, and since the pathogens within the plant would be isolated from contact with the free oxygen radical you're depending upon for resolution (no systemic efficacy associated with H2O2) it wouldn't work except in cases where some of the pathogens are on exposed plant surfaces, and then it would probably work as more of a speed bump than as a fixative.
Al
Soberdude
