We're having problem with image renditions and blob cache. When we upload images to a site image renditions generate properly. But if we change crop of an image, that rendition does not update. We have to clear blob cache to fix this problem. This problem occurs sometimes even in 10 minutes after we clear blob cache. I don't think clearing blob cache all the time is the solution. What might be the problem? Why blob cache is out of sync most of the times?
This reminded me similar problem with BLOBs like yours that I had once. I still believe that was environment specific error, because not only BLOB cache goes wild at this time, but at least you can check one thing called Configuration cache (if it is healthy).
Config cache is a simple folder located at %SystemDrive%\ProgramData\Microsoft\ SharePoint\Config\someGUID (ProgramData folder is hidden) and it is full of various .xml files with configuration changes. This folder must contain the same number of .xml files on each SP server in farm in order to keep settings updated.
In my case, cache was not synchronized between servers (folders were differed by hundreds of files) and must be "reseted" - Timer Service stopped, all .xml files cleaned, the same number in all cache.ini files inserted and service restarted. It is not hard, the only trick is to do this on all servers at the same time with Timer job stopped (= means short outage for users) and you can do it manually or use a script from CodePlex. And cache can be out-of-sync even if you have single-server installation only, even if there is no need to push configuration changes to any other node. It is just harder to discover the root cause.