OTOH, IPv4 extensively uses NAT, so that blocking the wrong IP may affect an entire network behind it. Traditional DNSBLs would be safer without such widespread use of NAT, which can be taken down using IPv6.
While on this topic, be aware the same could happen with IPv6 very easily. As people will no longer get a single address but a whole range, there will probably be some need to aggregate. As assignments per end site can vary between /64 and /48 you run the risk of blocking 65.000 customers while you intended to block one. To partially solve these issues there is now a attribute in de RIPE Database called assignment size, indicating the size of assignment to a single end site. If you are working on black- or grey listings you might want to have a look at RIPE-513 which describes the use of this new attribute. For your information, MarcoH (co-author of the document)