Black hole routing will possibly have a lower CPU/processing overhead on the router than a firewall. The firewall though allows you to be more granular with what you drop. At the network edge you have the choice of either.
As I see it the bigger benefit to black hole routing is in the core of the network where packet rates are far higher and the routers have no firewall functionality enabled. With black hole routing you can still drop traffic in hardware, and possibly at line rate.
Regards