Blue zones not interpolating in UFOs or OTFs output from a designspace

I am hoping to generate OTFs with FontMake, then auto hint them with `psautohint`. I have found that psautohint makes a pretty significant positive difference in the spacing of small text in PDFs ... but I find FontMake to be a lot more user-friendly than the AFDKO (or perhaps I’m just more used to FontMake, so far). So, I’d like to blend the two, if possible!

However, I’ve found that the `postscriptBlueValues` don’t seem to interpolate when I use FontMake to output UFO instances from a designspace. I would have to confirm it, but I suspect that other font info "postscript" items don’t interpolate, either. Just as a note, I would hope that if these _did_ or ever do interpolate, they would take into account axis mapping, so as to accurately reflect changes in outlines.

Is there a way to get these values to interpolate? If FontMake doesn’t have anything built-in, I will have to either make my own Python to do this, or else just use the AFDKO.

If there’s not a built-in way to interpolate these values, is it intentional that these aren’t interpolated, or more of something that just hasn’t been implemented yet?

Thanks, as always, for the awesome font build tools!