You have to export to QuickTime Movie, not QuickTime Conversion. You don't want to force any recompression on the video; it will only cause problemsin Toast.
If you want to save time & disk space, UNcheck the option called 'Make movie self-contained' when you export to QT Movie. As long as your Blu-ray authoring tool (eg Toast) is on the same Mac as FCE, you can use the QT reference movie produced this way as input to your Blu-ray authoring app, same as you would a self-contained QT movie.
It is important, however, that you make sure to do Sequence > Render All > Both and Sequence > Render Only > Mixdown before exporting your video from FCE.