When you said "inward block" did you really mean "vertical-outward" as this was the subject of your post? I'm guessing you did...I see the choice of block dicated by what stance you happen to be in. The majority of people would favour their right-side (strongest side) so the blocks would be with the right arm, from a right-neutral-bow.
Having said that I get the impression that there is a large variation between schools as to how the basics are actually performed and what emphasis is placed on them in a technique....so for some people an outward-extended might be preferred to an outward-vertical, purely because of the way they execute their basics, regardless of which would actually be the most effective.
In particular there seems to be variation as to how people perform the outward-extended...some use the "teapot spout" formation of the arm in a sweeping outward-horizontal motion, whilst others choose a vertical alignment in their arm. Understanding these two variations is quite difficult I think if one has only been exposed to one way.
So this is how I execute these blocks:
My outward-vertical block first rises up so that my forearm is parallel to the ground, my elbow at shoulder height. My shoulder then rotates outwards so that my forearm rises to vertical, at the same time my forearm also rotates outward, so that the palm of my hand rotates back towards me....all this time my elbow is inline with my shoulder joint.
I see this type of block being effective against a straight-punch as you say. "Unfurling Crane" uses this as the first block, at least the way I do it
I execute an outward-extended by first transitioning through an outward-vertical. I then rotate my shoulder outwards so that my right elbow is directly inline with both my shoulders (i.e. directly to the side of my body-structure) I then rotate my forearm so it is also aligned the same way as my body (i.e. little-finger farthest away), stabbing my forearm out to form the outward-extended. At the time my block connects with it's target (i.e. the attacker's arm for this discussion) my whole arm is in a vertical alignment, there is no horizontal motion at all - purely an outward+downward hammering motion.
Using the clock principle, I would say that my outward-extended is executed to about 1 o'clock. I would use this block against a hooking punch. My target for this block is the inside of the attacker's arm at the bicep, hitting this target straight-on at 90 degrees. I don't see this block being nearly as effective against a straight-punch as it would just glance off+up the arm towards the inside of the shoulder....hmm maybe this would be ok I'm not so sure now.
However I think the range of the attack would be a factor in what block you would use.
Taking a hooking-punch attack. If the attacker was very close I would use the outward-extended and block at the bicep, halting the attack so that the attacker's body-frame was wide-open.
If the attacker was further away my outward-extended would not reach as far as his bicep. And I would have to over-rotate my shoulder to block the forearm. So I would simply use an outward-vertical, blocking on the inside of the forearm or even at the wrist.
But really I think it really depends on how one performs these blocks, what range you are dealing with, and also what attack you perceive is coming next.
james