simple news confirmation message
You could try setting the subscription confirmation pages at: /admin/config/services/simplenews/settings/subscription > "Confirmation pages"


  SUBSCRIBE CONFIRMATION
  
  Drupal path or URL of the destination page where after the
  subscription is confirmed (e.g. node/123). Leave empty to go to the
  front page.


It may be that that page is only used when confirming from the email link, but might work for the subscribe block as well.

The message text could be overridden with the stringoverrides module