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Hi,
I edited my earlier post just to add clarifications (red font).
The dropdown menus in Blocks Admin are for viewing purposes. If you want to work with blocks that are accessible to Webmasters, choose from the dropdown. If you want to just work with blocks that are visible, then choose from that dropdown list.
Sometimes, when you've installed many modules, there can be many blocks in that view. So, you can use the dropdown list to narrow down the view so you can work on the blocks better...well, that's the theory behind those dropdown menus.
Default view is for
all blocks accessible to
Registered Users.
Hope this helps to clarify things. If you want to restrict access to blocks per group, you have to go to the Group Administration page.