Covering blocks with earth

Yes you can all you have to do is get a stack of earth or rock by digging the ground with your pickaxe i would say a couple of stacks at lest depending on how big you need to cover up and select it so its in your hand, you can also move the earth around by your arrow keys and page up / page down buttons to get a better finish if you so wish before you actual place it.

If you also want to cover it with grass just use your sickle on some grass this will add grass to your inventory which you can plant again.