Hi Dennis,
Puppet really comes into it's own when you've got a single identical change to make across a large number of devices, but it does mean a configuration change, which is only pushed out at a fixed interval (say 15-30 minutes).
I can't help thinking though that for your RTBH scenario, that something more agile like BGP Flowspec would be better suited to the task:
http://www.juniper.net/techpubs/en_US/junos12.1/topics/example/routing-bgp-flow-specification-routes.html
You could have a single router that operators log into to add prefixes too, then let BGP propogate that out to your edge routers, which instantly drop or rate-limit matching traffic.
Flowspec is also available on your M-Series today.