It sounds like your question is really just simple routing. Your R2 can advertise any routes about which it knows to R1, either through an IGP, iBGP, or both. R1 can then use eBGP to advertise the aggregate prefix to your ISP, which advertises it to the Internet. Any traffic on the Internet that is destined for any address in that aggregate prefix will be routed to your ISP, which in turn routes it to your R1.
When R1 receives the traffic destined for the aggregate, it will look in its routing table to decide where to send the traffic inside your network. Any traffic destined for the aggregate, but having no individual network inside your network, will be blackholed.
Your router will send traffic to the interface in your routing table that has the longest match. The aggregate black hole will have the shortest match for any networks you do have in your network, so it will only get traffic for which there is not a longer prefix. Basically, if the traffic is destined for a network in the routing table, it gets delivered to that network, otherwise it gets dropped.