A block level design is something which presents all the functional parts of your application in a visual format.
In case of Block level diagram of Android application you can start with android architecture
and then from the applications block onwards you can have your application, then as per the responsibilities make separate blocks for related activities along with their respective UI wherever applicable (UI is not needed in case of services, broadcast receiver etc) in xml file. And your block diagram is ready.
Same process you can follow for your required mobile platform.
Note that the block diagram gives overall view of application. so no need to have detail description of every aspect.