I have two paths. Both have separately been set to a blur value of 12.0.

If I select either and bring up the Fill and Stroke dialog, I see a blur value of 12.0.

If I select both together and bring up the Fill and Stroke dialog, I see a blur value of 5.5.

This seems like a bug (seems so from a user-perspective), but I sense I'm going to be
told they are two different filters, and ...

hmm... yes, it is a bit strange. I guess the only response i can give is that blur appears to be an extensive property. and is size dependant.

Thnks.. I just tested with a duplicated object and the problem sill occurs...

Oh, I sort of see what is going on. If I set them individually to a blur of 5, but
select both, the blur value shown is different depending on how far away the objects
are from each other (i. e., the total selected size). But if I then select one object
again, it is still 5.

Wht I want is to select a bunch of objects, and adjust their individual blurs
to each be the same value. My gues would have been this would be the more
useful way to have the UI work, but who knows what others think.

Ok, I think the problem is that the Inkscape developers thought that using the standerd deviation as the main blur setting was unfriendly, so instead used the percentage of width/height.

There is a work- around for what you want to do though:

1) set the desired blur on one object
2) go to the filters effect dialogue ( Filters-> filter editor )
3 select all the objects.
4) apply the guassian filter to all of them at once
5) change the "stranded deviation" property using the filter editor.

Thanks for the reply. Not sure I understand how to do step 5 (and maybe step 4)
in the filter dialog.