It's possible, but unfortunately a bit obtuse in this language (you might want to just use photoshop for something like that...).
One way you could do it is as follows:
OPERATE SET R = (IN[COL,ROW-1].R + IN[COL-1,ROW] + ...)/8;
Unfortunately, that's just the R channel. I should find a way to consider COLOR that would be smart enough to just have it apply the operation across all channels. (right now, the COLOR keyword is nice for simple "set color to this" type of things)
(not to mention correcting for gamma depending on exactly what you want to do with your blur... again, pixQL probably isn't the right tool for this as it currently stands hah)
You could even apply it to the mask resulting from the select. Mathematical morphology, advection using several pictures, interpolation, there are many use case that would be possible with a single query.
But maybe I'm too optimistic with the current possibilities.
Promising project anyway.
SELECT *: OPERATE SET COLOR AVG(ROW[-1], ROW[0], ROW[+1]))
To apply a 1 pixel horizontal blur?