I've not done much with blind testing partly because my interest (including time or budget) isn't at a level where I feel the need to seek upgrades that are not easily discernible.
I did a lot of A/B testing (not blind) with a set of Nordost Sort Kones under my SACD player in years past and thought that there may be some improvement, but never to a point that I considered that I've know if they were in or out in any type of blind test. My system is more resolving now, but none of my equipment or current cabinet was a good fit for the Sort Kones, so I finally sold them.
I've dabbled with some affordable cable upgrades (most expensive was about $250ish) and felt that the improvement was immediate and obvious when I heard a difference. I heard a clear improvement with two interconnect upgrades, but can really only say that there was an improvement from power cords on one of three components. I consider that the upgraded power cords should be better than what they replaced even though I didn't hear a difference, so I kept them on the outside chance that they might benefit from burn-in.
I've found that differences are often small when compared A/B, but with some extended listening there's a different level of enjoyment. My first "hifi" gear was an Integra receiver that I still have, so can compare to my current Pathos amplifier. In an A/B there's a clear difference, but the receiver does not sound bad to my ears, but if I use it for background listening can start to grate on my nerves over time.