Okay, FINALLY got to observe a test run on this.
A mandate to make short swords was violated, and the baron chose to blame a random mason. The mason was sentenced to receive one hammerstroke. The blind captain of the guard / hammerer successfully located the mason without any trouble, and dragged her to a chain. By default the blind captain/hammerer is assigned to carry a spear, so to deliver the hammering she went to the weapon stockpile, withdrew a silver warhammer, and hauled it over to the chained mason. ( Side note: she actually hauled the hammer, she didn't equip it.) The blind hammerer delivered the hammerstroke, and it successfully struck the chained mason's hooded-but-not-helmeted head. The blow badly injured the mason's upper spine and she quickly suffocated, presumably from paralysis.
This is only one data point, but previous observations of blind dwarves in battle have led me to believe that they have a quite small change of actually connecting with a normal combat blow. So:
1) Blindness doesn't interfere with a captain of the guard / hammerer finding their victim. No big surprise here, since blind military dwarves seem to be able to locate "kill" targets without trouble.
2) Blindness *probably* doesn't interfere with a dwarf's accuracy when dealing hammerings. More data points are needed to increase confidence. Since the blind hammerer used a "hauled" hammer to do the deed, it may be an indication that normal combat mechanics don't govern hammerstrokes delivered as part of the justice system.
2) Alternatively, perhaps the blind hammerer had a very high "chance to hit" multiplier since the victim was chained, which is sufficient to counteract the penalty applied by blindness. I consider (2) to be more likely.
3) It would be interesting to see how a "beating" delivered by the blind dwarf in her "captain of the guard" role would play out, since perhaps that action is less likely to be coded outside of normal combat mechanics, which seems like it may be the case for hammerings. Do "beatings" involve chaining the victim beforehand? It's been awhile since I've seen one inflicted.