Although I think KRyan's answer's correct that it's reasonable for the maneuver to be agreed upon by the table to work in a potentially unusual, it-can-sort-of-turn-back-time way—and I think that's valid, martial characters needing all the help they can get—, below is a harder-edged, rules-as-written way to read the maneuver in case you want an alternative.
The martial adept can employ the maneuver after the DM reveals the sum of the foe's die roll plus or minus any modifiers for the attack roll but before the attack either misses or hits and deals damage
The term result is used fairly consistently throughout the game to mean the total of a die roll plus or minus any modifiers. For example, the Player's Handbook uses result in just such a way on pages 4-5 under Playing the Game, although there result is mentioned with regards to a check's DC. It's not until the Combat chapter when the Player's Handbook on Attack Roll says
An attack roll represents your attempt to strike your opponent on your turn in a round. When you make an attack roll, you roll a d20 and add your attack bonus. (Other modifiers may also apply to this roll.) If your result equals or beats the target’s Armor Class, you hit and deal damage. (134)
Emphasis mine. And similar attack roll result language is used in the Rules Compendium section on Armor Class (15).
The 2nd-level Iron Heart maneuver wall of blades [counter] (Tome of Battle 70) says
When an enemy makes a melee or ranged attack against you, you can initiate this counter to oppose that attack by making an attack roll with any melee weapon you are holding. Use the higher of your AC or your attack roll as your effective AC against the incoming attack. You can’t use this maneuver if you are denied your Dexterity bonus to AC against your attacker.
You can decide to use this ability after you learn the result of your opponent’s attack.
Emphasis also mine. Thus announcing an attack roll's result usually determines whether a target either isn't hit or is hit and damage is dealt as that's when the target's AC is met or exceeded, there usually being no option to do anything between announcement and outcome. However, upon that announcement, a martial adept can employ the maneuver wall of blades so that the foe's result must meet or exceed the higher the adept's Armor Class or the attack roll generated by the maneuver wall of blades.
(As an aside, this is exactly the reverse of the process I use when I DM. I ask a player for the player's PC's Armor Class then announce if the NPC's attack either misses or hits and deals damage—never announcing the NPC's attack result at all—, instead of announcing the NPC's attack result and having the player tell me whether the attack misses or hits and deals damage. This feels just a little more right to me, probably because it impedes—however slightly—the metagaming necessary to determine an NPC's attack bonus. Nonetheless, I am unsurprised some folks play differently.)