Can Blender be used for post production visual effects?
Short answer is yes. Blender’s sequencing module can be used for editing, transitions etc. and some SFX like image enhancement.
You can mix and synch sound effects, background music etc. (Although the final multiplex of sound and video still has to be done externally) in the sequence editor as well.
The new Composite Nodes system will also greatly expand this by adding Render-Time effects to your work.
I have you used to ad rain, and lighting. I also have put video into blender for intros to shows and it is capeable of a whole lot more.