But it can easily function the same way as blocking perception.  It fits flavor wise, functions nigh on identically to a RAW example but with a different skill, and isn't really broken given these facts.  Especially given that the character is doing this out of flavor motivations.  In that sense it is really awesome.  I'd love it if my players were that creative...  granted I require justifications and personality reasons for any spell.  My non-offensive wizard uses mental blocks against moving on supernaturally strong foes frequently.  He has yet to break anything doing so. (granted my average roll of -2 breaks the campaign a bit, but that is just my DM dice.) 
I guess this is one more flavor over function argument although I would argue it is RAW, just not specifically mentioned.  And filled with Flavor, which strikes me as the name of the game here.  And I would say that once the type of block is resisted it can't just be put back up again the same way.  They have overcome their fear of ghosts for the scene kind of thing...  But that is once again a flavor call.

With a DM handling things right that should never happen.  Technically yes the DM should choose what they resist with, but if creating a fear based effect the obvious answers are discipline or conviction.  You always choose what they resist with in a sense.  They resist with something that dodges/blocks the effect.  Dodging fire requires athletics or a shield.  Dodging fear requires willpower or discipline.

Admittedly Blocks always hit, and against weak willed individuals a mental block is a real pain, same as throwing  a physical block against a weak person would be.  That's good use against those opponents, exploiting their weakness so that just throwing a horde of mooks doesn't stop the enemy.  Now throw that same thing at a fetch.  The fear based creature is just going to laugh, it doesn't know fear(likely a tag).  Throw that same spell at John Marcone and he'll make his roll.  Once they break the block it is gone and they continue on with what they do. 

Considering the character is Sherlock Holmes given 3 pages of reading the general justification is, "Because that's what Holmes does."  That said, the idea is pretty brilliant.  I've been debating doing some kind of Holmes referencing somewhere in my Campaign.

I enjoy crossover ideas and thought I'd throw this out for a quick balance check.

Staff of the Word(-3)
Item of Power given to Knights of the Word by the Lady of the Lake.
Gives
Sponsored Evocation as per the ability Evocation
Sponsor requires that the user be on a mission for full effect.
+1 to lore, discipline, conviction while casting.(In reparation for no item slots.)
Can be tagged as sponsored 2 times per scene, 3 times max per session.

Is part of the Knight of the Word Class which requires
Emissary of Power(-1)
Gives +1 to all rolls versus demons while on a mission.
The Sight(Limited)(-1)
Modified Cassandra's Tears(-0)
-Apocalyptic Dreams showing Consequences of failure.

For Reference this comes from Terry Brooks the Word and the Void Series.