<p>Is there a way to manipulate the speed of the video playback? I'm especially interested in a way to slow down with frame blending, exactly like the function in Final Cut Pro. </p>

Interpolation should be able to help you. There's an example included with Quartz Composer (Interpolation Modes.qtz) and a beginning tutorial
here
that breifly mentions it (step 5).
this wiki article
also discusses it and talks about the different types.
Note: I don't actually have a Mac that can run QC, so this is just what I've been able to find through Google, but it sounds like it should get you on the right track.