Darkened oil is absolutely a sign that the fat has deteriorated. Constant changing temperature, breading particles, protein juices, water (the worst) and anything else that goes into that oil starts to break it down. By time that fat smokes, it’s really bad.
To extend the life of your oil and keep the foods you fry in it tasting neutral (no after taste), you need to filter it daily. There are some good additives that extend the life of the fat. Most places like Sysco sell them. They have some ascorbic acid in the mixture..I have no idea what good that does.
One of the most frustrating things we use to find was everytime we changed our oil (we had a built-in filter system) the fresh cut fries we used would never look completely fried. So, if someone wanted them well done, it took forever. As the oil grew older, the color of the fries improved. Then when the oil got too old, the exterior color would darken so fast that you had to make sure the inside of the potatoes were actually cooked through.