My friends and I have wondered about this one since HC came out since we dont play competitively the answer never mattered.

The Line of Fire passes through an exact diagonal, one square of the diagonal is clear terrain, one side is blocking terrain. Is the shot blocked or is this clear LOS? I am sure this must have come up when the first HC map came out with the four street corners

If it crosses on the exact diagonal then it is clear LOS.

I'd have to say hindering. However, if the target had stealth or a team ability granting stealth, then it would be considered blocking.

Oops I mistakenly thought you said hindering and blocking. It is clear in that case then.

This is from the recently posted FAQ's provided at the home page of this site:
"Lines of fire that pass through the diagonal of diagonally adjacent squares containing two different types of terrain are considered to pass through whichever terrain type is more advantages to the attacker."