Sirius is an incredibly bright star. So much so that it completely over-exposed the film plates used by the Palomar Sky Survey, which is where the "censored" image comes from. The "censoring" was to remove or block the part of the film emulsion that would expose the central part of sirius. The diffraction spikes and some of the glare is still there, but you can make out far more stars closer in to Sirius than you otherwise would. Bear in mind this was before the days of CCDs and digital imaging, so they couldn't just shut off individual pixels precisely and cleanly. Doing this though allowed fewer stars to be obscured by the glare of Sirius.

Here's a comparison image I made using the original source data from the individual film plates, which you won't find at a secondary source of sky survey data like Google Sky. First up is the familiar "censored" image:
[link to i319.photobucket.com]
Now here's an "uncensored" image using the same telescope but with film plates that did not obscure the light from Sirius at all:
[link to i319.photobucket.com]
Note how many more stars are drowned out by the light of Sirius in the second image.