Canon EOS 6D Instruction Manual page 68

A Fully Automatic Shooting (Scene Intelligent Auto)
The shutter speed display is blinking.
Since it is too dark, taking the picture may result in a blurred subject
due to camera shake. Using a tripod or a Canon EX-series Speedlite
(p.170) (sold separately) is recommended.
When the external flash was used, the bottom part of the
picture came out unnaturally dark.
If a hood is attached to the lens, it can obstruct the flash coverage. If
the subject is close, detach the hood before taking the picture with
flash.
In Basic Zone modes except <G>, using a Canon EX-series Speedlite
(sold separately) in low-light or backlit conditions can capture the subject
brightly while avoiding camera shake or reduce backlit shadows.
68