Didn't stump me.

It's not the borax.

Are the endcaps on the filter by any chance blue? I have seen the rubber endcaps turned into 'goo' by very low pH more than once and you descrbe a VERY low pH situation that was present for probably a long time.
Check the endcaps (both of them) and see if they are soft and/or sticky. I bet they are! I also bet there is blue 'goo' all inside the filter chamber.

The only other thing I know of that will cause blue goo to collect in a filter are some clarifiers. Have clarifiers been added to the pool? (although my money is on the endcap being damaged by very low pH.