An easy test would be to remove your lens, power on the camera, and navigate to the menu that lets you activate the manual sensor cleaning mode. That mode tells the camera to raise the mirror and open the shutter -- and it just holds it open so that it can be cleaned.
The reason you should do it, is because it will let you look into the camera and see if, in fact, the mirror does raise up completely clear and if the shutter opens completely.
If not, the camera needs service.
Shutters are the one part on a DSLR camera that actually is expected to fail (it's considered normal). But it's usually good for somewhere around 100,000 shutter activations (and is often much more) even on an entry-level camera and many high-end models are rated for significantly more.
If you use the flash, then there's a "sync" issue. The flash is very fast ... often being illuminated for only around 1/1000th sec. But the mechanical shutter can't actually open & close that fast. On your T3i I think the fastest "sync" speed for flash is only 1/250th sec. This gives the shutter time to completely open... then the flash can fire ... and then the shutter can close. If you use faster speeds than the camera's rated maximum flash-sync speed the shutter wont actually be completely open when the flash fires and any part of the image that wasn't exposed when the flash fired will be black (or at least very dark).
In your image, the black area appears to be completely black (not just very dark). This makes me suspicious that your shutter is sticking and that it wasn't a flash exposure at a speed faster than the flash-sync speed.
Tim Campbell
5D III, 5D IV, 60Da
