Defensive duelist was picked by a battlemaster in my OOTA campaign, in late tier 2 I think it was. It was extremely strong.
It is a lot better at higher levels. In tier 4 it is equivalent of an at will shield against 1 attack a turn. Shield is stronger but uses a slot.
I think this is one area where people focus too much on relative impact instead of absolute.
As an example, take a character getting attacked 6 times in a combat with a 50% chance to be hit. On average he will take 3 hits. Blur would reduce it so that he takes 1.5 hits. That a 1.5 hit difference.
Then look at an example of a character getting attacked 6 times in combat with a 30% chance to be hit. On average he will take 1.8 hits. Blur would reduce it so that he takes .54 hits. That's a 1.26 hit difference.
yes but that is a huge difference in relative damage taken and it also is the difference between getting hit and not getting hit and other effects besides damage that often come with being hit.
For example getting hit one time less by a Ghoul means 1 less save required to avoid being paralyzed in addition to taking less damage over the course of a battle. With a 0.54 hits per fight most characters will probably not have to save at all. This pays forward too, by preventing the hit and the save to start with it also prevents future hits because if the Ghoul paralyzes on his third hit then the target with no disadvantage will likely get more than once more on the other 3 attacks later in the fight. If he gets paralyzed on the first, he is probably going to take 2 or 3 more hits. A Ghoul is CR1 and 3 hits with 2 criticals is likely going to kill many 1st level characters outright.
When you consider damage you need to consider critical hits. Using the same target 15 to hit, one in every 6 hits is going to crit and if you fight 6 times a day you are going to get crittted once or twice on average. With blur that is a flat 1 in 400 attacks regardless of AC.
Finally this presumes you need a 15 to hit (30%). The math is exponential and because of that Blur is most effective when your base defense is already better than that. We had a guy who played a bladesinger in our game and went multiple levels without getting hit in combat at all (like levels 4 to 7), not a single time and she was the first into melee every combat. Some of that was luck, some of it was the DM choosing not to attack her, but most of it was her base AC that was 22 in bladesong (mage armor, staff of defense, bracers of defense) before the shield spell and she had the lucky feat, for the already very, very rare case that the enemy rolled high enough on two dice.