Why is it recommended to blanch onions before pickling them?
From  indiacurry.com :


  Blanching Vegetables for freezing or pickling
  
  Vegetables have a natural enzyme that continues to effect texture, color and flavor. Blanching stops the enzyme action
  The natural enzymes help vegetable to grow and mature until they are harvested. After the vegetables have been harvested, they continue to remain active even when frozen or pickled making the pickles to be tough, effect the color and flavor. Blanching stops the action of the enzymes.
  The purpose of blanching is not to sterilize or pasteurize vegetables. It may kill some but not all the surface micro-organisms
  
  Blanching is done by either scalding vegetables in rapidly boiling water, or in a steamer for a short time period. The vegetables must be scalded just long enough to stop the enzyme reaction, but not too long to make them soft and mushy by breaking up the cell-walls.
  Under-blanching stimulates enzyme activity and is worse than not blanching
  Over-blanching causes loss of texture, color, flavor and vitamins.
  Vegetables are blanched for freezing or making Indian pickles.