I don't have a tank class leveled so can't test this for myself, but my FC mate did some testing for me and says it appears the parry and block range is about 120 degrees favoring the sword arm. Obviously this is anecdotal and would need further testing. Wolfandre here says his testing indicates cone is even more narrow at 90 degrees.

I've been advising newer tanks to position mobs in front and don't want to give them wrong information on how to optimize their parry and block skills.