Armor is intended to block the power of an attack, a block is always blocking the skill.
I think the intent behind evocation blocks being linked to the power is to have the caster make a choice between the power they want to draw up and the safety the block is supposed to give them. Too much power and they hurt themselves more than the block helps them by taking casting stress and backlash, too little, and they cast the spell for nothing. You don't get the randomness, positive or negative, that rolling a skill for a block would. And you paid a good amount of refresh for it. All in all, I think it's fair.
Though I have to say, I never liked blocks the way they are anyway. I like the way Fate Core handles it: You do a maneuver (or as Core calls it, you "create an advantage"), and as long as the resulting aspect exists, you can use it to justify active opposition. Active opposition means that someone is trying to do something, and you can actively defend, instead of just having to roll athletics to dodge. The skill you use depends on the aspect you create. So Harry could do a "Force shield" maneuver, and then use his discipline instead of athletics to dodge. Or, if he extends the shield to Murphy, he could roll his discipline instead of Murphy rolling her athletics, when she is attacked.