I think that it is healthy that the ecosystem has more services that provide down voting. But if you want to include reasons such as "Blockchain tells me you’re circle jerking/selfvoting" or even "From my perspective this post doesn’t deserve that much value" you need to create a super transparent system. The downvoters should always sign that the action has been done by them and for which reasons. Also the downvoters should be those of a superb reputation in the system (oh how we could use a working reputation system right?:P)
But if you decide to go with the downvotes you need to accept that the service will be controversial – because human psychology and the effect of negative incentives on it.