Can't get suggestions for Dotted Blue Underline in OS X Dictation
aThe blue underlined words (if it works in the same way as it does on iOS (I don't use dictation on OS X)) then they are not there to provide alternate suggestions, but they are there to tell you that OS X is not entirely confident in its transcription. If a word is underlined in blue, it's saying that that was it's best guess, and if you select it you can edit the word if it is wrong in order to correct it.

If you do, Siri / OS X Dictation learns to connect that word to what you just said, and thus becomes more accurate after this is done tens of thousands of times.