It could be the new version of the bleach being carried over, but whatever it is, it is the result of insufficient washing. Chemicals of one sort or another are being carried over.
C41 has several bleaches out there ranging from blood red to deep green.
I made the assumption that it might be the new green bleach appearing blue in the stab. The conclusions remain the same. Carryover of chemicals into the stab. due to insufficient washing.
The only liquid I know in E6 processing which turns blue like that is Colour Developer, and this is not a liquid which should carry over into STAB unless something went completely wrong in the process. Whatever happened here, just replacing STAB will not solve this.