In the Ning Import Config tab (Import Filters section) is a 'Import Directly from Ning' checkbox. If not checked the import process will use the '-local' json files. If not, it'll use the other json files and try to grab all media directly from your Ning site.
Haven't done a Ning import in a while but do remember that I nearly always checked this box as there you only needed to upload the json files to the Jamroom server.
Maybe try checking this box just for blog imports?
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Paul Asher - JR Developer and System Import Specialist
updated by @paul: 09/18/23 06:19:34AM