Search for specific blocks

Hwo do I search for specific blocks in all worlds ?
I got to this point:

Code:

    	WorldListener eventWorld = new WorldListener()
    	{
    		public void onWorldLoad(WorldLoadEvent event)
    		{
                        // event.getWorld().???

    			getServer().getScheduler().scheduleSyncRepeatingTask(plugin, new Runnable()
    			{
                            public void run()
                            {
    	            	           // ???
                            }
    	                }, 100, 100);
    		}
    	};
    	
    	pm.registerEvent(Event.Type.WORLD_LOAD, eventWorld, Event.Priority.Monitor, this);

Or am I going this the wrong way ?