* Prompt means to encourage or cause someone to do something, or to be done quickly.
* Blunder means to make a careless mistake.
Here are some examples:
Prompt:
* The teacher's questions prompted the students to think critically.
* The deadline prompted me to finish my work quickly.
Blunder:
* He blundered into the wrong room.
* I made a terrible blunder in the presentation.
While a blunder can be caused by someone not being prompted to do something correctly, the words themselves are not interchangeable.