This is almost unbelievable but since near a month with my 5DMKII today was the first day I went out with it to grab some pictures (somebody please kick me, I deserve it LOL). While this is my second 5DMKII, I sold the first one with very very short use, actually one corporate job as far as I remember, and also never went out with it (yeah I deserve another kick). So, that said this is my first time and I notice this blackness at the corners, it was taken wide open with the 24-105L. I put on a filter over the clear filter (don't ask, I just did) maybe thats the border shown here or was it the lens hood? (the images is cropped so I can upload it, but the same appears at the four corners).
FYI This only appeared at this pic, I took over 75 and at none of them I see this. And this was the only pice Wide.
Here the Exif:
1/1000s f/4.0 ISO200 24mm
Thanks for looking, and I believe this is my first pic posted, jeez finally!