At which point this solution degenerates into the same solutions that already exist, with the same problems. Take voter ID cards. Lots of people don't like them because they say it disenfranchises people who would have problems acquiring them, like the poor. A digital signature is going to have the same arguments.
But, I agree: that's not the point of the block chain, its ancillary. But involving the blockchain adds about as much towards solving the real problems with voting as saying "Hey! What if wrote down the votes, but not the person!" Ballot stuffing is about all the blockchain solves.