If the blix is blood red, and the pH is between 6.2 and 6.8, and if there is no apparent sulfurizaion (like bad hypo), then something else is going on here. It is probably too dilute or too exhausted somehow. Exhausted or dilute blix retains its pH and color but just won't work.
If the pH goes above 6.8, the activity will fall off very rapidly as well but for another reason. The oxidant loses its activity above 6.8. Below 6.2 the fixing action slows and the fixer tends to decompose.
If the blix is turning orange, the pH is getting too low due to carryover of the stop into the blix. If the blix is getting cloudy, then the low pH or some other factor is causing decomposition of the ammonium hypo in the blix.
You can often fix this by aeration and then addition of ammonia and ammonium hypo to ghe blix. This regenerates the oxidant and replenishes the hypo. Adjusting the pH increases activitiy.
You can test blix with B&W paper by fogging B&W paper and then developing it in Dektol, stop, fix, wash, dry as normal. This black sheet of paper should blix to a good white in about 2 - 3 mins at room temp in good blix.
You can test the theory that your blix is bad by reblixing and washing some of your prints with a grey dmin. If it vanishes, then it was indeed the blix, but if it does not, then it was either fog or bad developer.
PE