After asking specifically to lift the blacklisting for my IP, they did. It now works.
I figure that what happened was: their server was down an extended period, and during that period our mail clients had a high number of failed connections, triggering the blackhole filter.
The thing is, DH ought to clear out those filter lists automatically after getting the servers back online. We shouldn't have to ask for that. Also, if we don't ask for it, the blackholing remains permanent, no timeout.