What may have caused Blue Staining on old Ilford FP3 120 film?

My day job involves working with collection material, of which quite a bit is old photographic items which include glass plates, film and prints.
These items have been collected over decades (we have glass plates dating from the 1800s) and the collection includes nitrate photographic film and older versions of various film stocks.

This particular negative caught my eye yesterday, as it has distinct blue staining, which I've never seen before. It is 120 film, Ilford FP3 Panchromatic film possibly shot in the mid- late 1960s.

Can anybody shed some light on what may have caused this?
TIA

My guess is that the blue coloration is remnants of anti-halation dyes in the film. This suggests the film didn't clear very well to begin with, potentially due to poor fixing and/or washing.
Development was definitely pretty complete though; those negatives are bullet proof, LOL!

I've seen this once with a roll of 120 FP4 exposed and processed in I'd guess 1975, fixing and washing was typical for all my films back then, and there was no blue stain at the time. I only noticed the stain many years later, and it's quite strong. I've shot hundreds of rolls of FP4 and never seen it again.

Maybe the anti halation dye hadn't been fully removed, and remnants aged.

Ian

I with koraks on this one. If you are able to, cut off a bit of the leader, and soak it in water for a while. If you have HCA, try that.