I've recently received a few requests regarding blackholes.intersil.net. According to this message to news.admin.net-abuse.blocklisting, this blocking list was retired in July, 2006.
Confusingly, the zone still contains a test entry. Regardless, I've seen no hits against it in a very long time. Therefore, if you are using this DNSBL for spam blocking, I recommend you cease doing so.
If you have any further information about this list, please feel free to contact me.