Seems implementation specific. BIND does this you're right, but many services don't follow the rule as closely.
Actually, from RFC 1035, section 5.1:
"If an entry for an RR begins with a blank, then the RR is assumed to be owned by the last stated owner."
No, a blank name uses whatever name was used on the previous record. Near the top of the zone file, that name is usually the domain name because of the SOA record. But later on, it won't be.