I've heard this about combined Bleach-fix before ... that it tends to leave a sliver residue.... but no one can ever explain where the silver would come from - or why it would be more prevalent with a combined "blix" than separates ( Blix
form the data sheets) contains a ferric compound - no silver whatsoever.

Careful with shortstop - Tetenal and Photocolor both advise against its use between color developer and blix. I do NOT use it - nor a water rinse.