Blanching works by denaturing the enzymes, rendering them inactive.
Enzymes are proteins produced by the cells to perform various biological functions. Their effectiveness is critically dependent upon their shape. They act as catalysis to speed up chemical reactions. When you heat a protein, you cause the hydrogen bonds created when the protein is folded to break, changing the shape of the protein. And since enzymes are proteins, their shape and therefore their effectiveness also changes. So by blanching, you destroy the effectiveness of the enzymes that would cause the food to deteriorate.
