Found out about this very recently when trying to work on a new blackhole route server; apparently in August 2012 a new RFC, 6666, was published specifically to define a newly allocated IPv6 prefix as being for use in remotely triggered packet discarding/blackholing.  Here’s the link:

http://www.faqs.org/rfcs/rfc6666.html

The new IANA-allocated prefix is 0100::/64

I had previously been using the IPv6 documentation prefix of 2001:db8::/32 for my RTBH (Remote Triggered Black Hole) setups since it requires a route you’ll never see in legit traffic to/from your upstreams.