Is it common practice to hide military bases like that or is it a special treatment?
Plus what the ... ar those things:

You'll also find things like the roofs of buildings in DC, etc. blocked out. Post 9/11 legislation to classify such things, in the name of
preventing easy access to such info by terrorists. If you look though, you can find plenty of pretty recent sat photos of Area 51, including many new
buildings and runways....