You might be able to get the water collected in the pit to clear up if you run enough clean water down the drain. Sooner or later the clean water should wash the sewer water down the drain. At least you would be able to see what you are doing. If that doesn't work you could get the line jetted to clean it out.

You could also put a small pump in the bottom of the backflow housing to keep the water pumped out while you replace the internal parts.