I agree, blockchain is an amazing technology that has potential to revolutionize voting. But, the thing that secures Bitcoin is the fact that coins are created only one way, and every coin (or fraction) used in any transaction is traceable back to the transaction which created it.
The problem with blockchain for voting is anonymity and auditing.
Let's assume a few things about a blockchain voting system. First, one must register to vote (join a union, visit a BoE office, buy a share of stock.. the context doesn't matter here). Let's assume that you are registering somewhere, and the authority records your name on a list of "registered voters", AND then uses a voting system program to generate a printed card for you , containing a scan box that encodes a random "secret key" that is now yours. The system might record that SK (but not your name) on a list of "SK already issued" (to prevent the infinitesimal chance of random duplication), or it might keep no record. On voting day, you show up, identify yourself, sign in.. the clerks verify that you are registered to vote and give you access to the voting machine. The system voting machine scans your SK card, generates "public keys" from it, and records each vote you make in conjunction with a PK which demonstrates that it was cast by the anonymous holder of a valid SK. (here, "valid" assumes that the machine checked the scanned SK against the list of "issued SK", or it assumes that any SK scanned by the machine is valid because it was presented by a verified registered voter). The machine can print a record on a backup paper roll, and can even print a receipt for the voter showing their votes and PKs. Probably, the last thing the voting machine should do is put the voter's SK on a list of "expired SK", so that card (or a copy) won't scan a second time.
That should work to secure your vote - the votechain is a public ledger, you can see all the votes cast and the PK associated with.. You can audit your own votes, comparing your own printed receipt to the public record, to confirm that your vote did count the way you voted it. THEORETICALLY, it becomes possible for a large group of voters to audit a vote by bringing in all their receipts. ("Theoretically", because you'd need more people to show up holding "I voted for declared loser" receipts, than the total number of votes credited to "declared winner". But, not completely impossible..)
BUT... What about "ballot box stuffing"? What stops the people holding that SK generating card printer, from printing a few thousand extra cards? What stops them from casting votes for themselves with these cards when nobody is looking? With multiple voting locations and machines, what stops them from keeping a few "spare" machines in their backroom, turned on and taking votes? Because of the anonymity necessary for voting, and provided by blockchain, it actually becomes fairly easy for voting authorities (and crooked system designers) to stuff phantom votes into the final count. To invalidate even one fraudulent vote, you would need EVERY voter to show their receipt to prove "i did not cast this vote". Only then could investigators start asking "how did THAT vote get into the system?"
In many ways, blockchain could make voting better and more accurate. But, widespread overconfidence in its "infallibility" would actually make voting easier to manipulate and ultimately worse.
SORRY for the length here - I wrote all this in a (feeble?) effort to ensure that it never happens.
