I simulated an attack by running this:

fcli set blackhole 10.0.0.99

And it announced 10.0.0.0/23 to the routers. I then simulated an attack on another IP with this:

fcli set blackhole 10.0.0.100

No new announcement was sent to the routers as this is in the same /23 already announced. I then removed the ban on ONE of the IP's (10.0.0.100) but FastNetMon withdrew the /23 announcement via bgp which meant that 10.0.0.99 was still showing in the blackhole list of Fastnetmon but it wasn't announcing the /23 to the routers for mitigation any more.