SmoothFreezeFrame
From Avisynth wiki
Like FreezeFrame, but blends all frames inside the start-end range together to create a smooth result:
function SmoothFreezeFrame(clip clip, int start, int end) {
replace = clip.trim(start,end)
smooth = replace.TemporalSoften(round((end-start)/2),255,255,255,2)
insert = smooth.FreezeFrame(0,end-start,round((end-start)/2))
return clip.Trim(0,start-1)+insert+clip.Trim(end+1,0)
}