It seems that Blackpoint is not just cleaning up the workflow history for processes after 60 days, but that it is also removing the "Completed" field on the library / list that the workflow was run on. How can this be fixed?

Here's the workaround: http://technet.microsoft.com/en-us/library/cc298800.aspx. And, for the record, SharePoint is doing the cleanup, not Blackpoint.