The sment Blueprints are really just zip files containing the folder in which the blueprint of the ship is stored. (look in your game directory for the "blueprints" folder.)
On the other hand, if you never saved the blueprint, all is not lost. Look in your game directory for the "server-database" folder. Under that, look for the name of your "world" (you might only have one entry in that folder. My builder world is named "world0" with a zero at the end.) Under that folder, go into the "Data" folder. Look for the ENTITY_*.smd2 files with the same names as your ships and copy those to the new, uncorrupted world Data folder. Don't forget to copy over all related rail entities!
Edit: Also, don't forget the related XML files in the world folder, before the Data folder.