I've read somewhere that when you add a little magnese dioxide (easily obtained from an alkaline battery) and mix it with hydrogen peroxide it flash boils (but isn't hot ?) (low boiling point) and then a bi-product simply black staining peroxide.

Perhaps you meant manganese dioxide? If you add it to hydrogen peroxide, it acts as a catalyst for the decomposition of the peroxide.

yes i did, my mistake.

the falsk would boil off (provided that conc. H2O2 used) because MnO2 catalyse the decomposition of H2O2 to H2O and O2. The reaction also releases heat which further accelerates the reaction. At the end, you are left with MnO2 (no change to this guy as it is catalyst) and water.

Silver also catalyzes the decomposition. It's often more convenient that manganese dioxide, especially if it reacts with the zinc when you try to get it out of the battery...

So does copper

And, so do many other transition metals and noble metals, starting from iron, cobalt, mercury, platinum, and so on.

also potassium iodide