EOS Rebel T3i When I look through the view finder I noticed dark blue coloring in the corners

[lcparker]

When I look through the view finder I noticed dark blue coloring in the corners.  Do you know the cause and fix?  The camera is a EOS Rebel 3ti.

 

[jaewoosong]

Are you using any filter/filters? on the zoom lens?  Just like what others have mentioned, hood could cause the vignetting but I've had a friend that also stacked 2 filters on top of each other which would also cause vignetting.  If you have any filters, try shooting a picture w/o them on.