Doing blacklisting of IPv6 addresses is actually quite hard, because you can't know how network owners lay out their addresses. Some networks (especially VPS providers) will use one /64 (or so) per datacenter, while other will give out a /64 for individual users.

So when 2001:db8:a1:b2::c:3 has sent spam to you, you can't know if you should block the network 2001:db8:a1:b2::/64 or the specific address 2001:db8:a1:b2::c:3. Some blacklists (carelessly) take the former approach and block entire datacenters.