I'm the Community Manager for FeedBlitz. I have two requests / suggestions.
Please give users the ability to not cache their feeds, feed caching creates a host of issues for our service and since we serve their feeds it creates unnecessary redundancy.
As an alternative, create an exception for the FeedBlitz user agent.
Please declare that a feed is cached in the comments for troubleshooting purposes. I've run across your plugin several times in the recent past as users complained about mailings not going out on time, only to discover the feed was cached and set for 7 days.
I would love to discuss this issue in more detail, if you have any questions. Heather at FeedBlitz dot com