What parts are blue: image [as opposed to black]; highlights
[as opposed to white]; or paper?

Ferricyanide bleach [aka Farmer's]?  The stuff eventually
forms 'Prussian Blue', as found in artists' paints.  Though
if it wasn't washed out I would think the image would disappear
before the whole mess turned blue.

Blue toner is almost [utterly] impossible to remove from
plastic trays.

Unwashed selenium turns purple-brown-red.

P'Fibre fogs fast, fwiw.  But that's grey.

Anything different about the half of them
that didn't turn blue?

 Blue, blue, my prints are blue,
 Blue are my prints and I'm without clue."

                           Ole Blue Eyes.

  This is very odd. Of course, you can see if re-fixing
removes the stain. Sometimes Iron in the water can cause
blue stains but this is usually confined to certain types of
toning.

Since it's old paper, it could be the silver oxodizing. Old papers very
often end up with blue prints. Maybe some were exposed to more humidity
than others. Just an idea, but what color blue?

Probably not adequately fixed though that usually takes days or even weeks  
to exhibit any real problems. Also it usually runs to a yellow rather than  
blue. Note also that adequately fixed DOES NOT mean that you need to fix  
longer. The fixer may be exhausted or simply compromised.