I hope the following is clear enough:
The thumbnails are the little pictures that should appear in the project "strip", instead of the "blue frames" you were having. These thumbnails are generated by FCP X automatically and stored in a specific folder inside the project folder.
For some reason which is beyond me, some of these little files may have become corrupt, and would not display properly - hence the "blue frames".
Deleting render files you also deleted the corrupt thumbnails, which forced FCP X to generate fresh new versions of them.