I don't think that PD has that feature. If you can point me to anything saying/showing PD's frame blending motion blur I'll be happy to see what I can quickly learn about it.
There is an interpolation setting when slowing down a video in the Video Speed Designer, where PD can create a middle frame based on the frames before and afterward, but as far as I'm aware there's no tool or setting to specifically blur skipped frames when using high fps clips.
You might be able to use a blurring FX set to a very low value over the entire video, just enough to take the sharp edges off the fast moving objects without being too soft when things are steady.
If you're willing to share a short sample 300fps clip, I can see what other tools or effects might help. Please see this
FAQ if you'd like to do that.