Oh... do you mean shooting the photo HORIZONTALLY and VERTICALLY?
For your very first post about blur in "vertical" shots...it might be your holding position not comfortable for your to hold the camera properly. That's why you get the shaky output (blur).
And the test you did. Without Flash is what it is causing the blur also. Your camera did the compensation by setting a longer exposure (slow shutter speed) and it's so easy to get camera shakes.
nth to do with flash.
but of cos, if u use either the built in flash or ext hotshoe flash, they will enable AF-assist light to emit, thus enable the camera to achieve focus (continuous green soild circle in VF)
EOS 7D|10-22|17-55 2.8 IS|50 1.4|85 1.8|70-200 f4L IS|580EX II|2x YN560|190CXPRO3+496RC2