We normally use AE for frame blending on speed changes.
we are editing in FCP, I don't think you can do this in FCP
I am guessing the option is too export to Motion.
if so what is the difference between 'optical Flow' and 'motion blur-blending'
Basically Optical Flow is a better way of doing speed remapping. The Frame Averaging method, as the name suggests, averages adjacent frames together over the length of a shot to create the in-between frames and uses a sort of mix/dissolve as well.
Optical Flow, on the other hand, actually uses motion vector analysis to compare two adjacent frames, then compiles a list of motion vectors that will create the in-between frames. It's more realistic and definitely better for fast action shots.
"Captain Mench," i.e. Michael Mench actually has an Optical Flow overview in the Shake tutorial section here at the Cow.