I know this was requested a year ago already but maybe it got buried, and I think it’s quite an important feature.
(and probably quick to implement, it’s just blur+add existing mask)
my dreamy vision: 
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That would be a fantastic thing, but unfortunately I don’t think that will work. It would require something like a +/- offset and such an offset does not seem to work. But let’s wait and see.
In ZBrush “Grow mask” is just a blur that adds back the existing mask, no radius or offset or anything. That would be more than enough.
So it’s almost like a normal blur but the important difference is that already masked areas don’t become any less masked, which is what happens with the normal blur.
demo normal blur:
demo grow mask:
Btw Zbrush shrink is also just invert mask → grow mask → invert mask