This would make most sense for you as this would be an incentive to open an account on blockchain.info. Up/Down-voting could be done by "donation" addresses which again would be good for you.
As long as you show the amount of donations as justification to rate something high, you should not be blamed for misrepresenting stuff and upon clear abuse you could close accounts or users and mark the tag as coming from a closed account. Showing tags from closed accounts last, only optionally or not at all would also be very clear to the user.r liability.
For now a tag submission page is available at
http://blockchain.info/tags. Also for a bit of geeky fun I indexed the addresses contained in signatures from bitcointalk.org example:
http://blockchain.info/address/1H4sGgNSRvRuiUsbucsQBNFWhZcLnL2KW1It doesn't include up or down voting as the tags in this context probably serve a slightly different purpose than your original idea giszmo. Which was to allow people to tag addresses as belonging to scammers, add comments etc? While I could see this being useful in some situations it has the danger of morphing into a blacklist which in my opinion is a bad idea all round.
This service should used every WordPress bloggers for micropayments ,donation & Crowdfund blogging.