He shouldnt have many 8x hands to defend 3bets with, but maybe depending on the rather small 3bet, he peels lighter. Weird river lead given that he cannot have the nut flush. I think his value range consists of KQ-KT dd, but he might be playing AK with the Kd just like this too.

76s is very unlikely to call turn with no SD value and poor pot odds. it just loses to your bluffs even if you give up on your bluff. He also shudnt have many flush draws that peel twice are Axdd hands with SD value. For similar reasons to the 76s, i dont think he can peel twice with T9dd because he just loses to your bluffs even if you give up, and its not like he can use to check/jam river because the blockers of T9dd are terrible and the SPR won't be large enough.
So it does look like his value range with be KQ-KTdd that make a lot of sense on each street leading up to the river. I dont necessarily agree with his river jam, given you will a good deal of flushes yourself as well as hands like AA/AK that have his bluff catchers in terrible shape. As a result, I think he shud be checking mostly, in order to make it so you cannot just vbet him to death when he checks this river and signifies he most likely has a weaker bluff catchers like 65s, 77,99,TT or KJs or something.
Regardless, you do a hold a relevant blocker to much of the value range he's repping (Axdd and Kxdd), but the issue is that i dont see what his bluffing range is. Does he randomly shove like 9x9d here? I would think some players wud use AdX as a bluff, like AdQx maybe, so you may be blocking his bluffs.
He only seems to have 4 value combos (KQdd Kjdd KTdd 76dd) as well as maybe the occasional full house, so all you need to do is find like a few random bluffs for this to be a call. He may perceive his value range as these 4 combos + AQdd AJdd ATdd A9dd, so he may have stacked on too many bluffs. In addition, I'd think he wud check some of these flushes to protect his checking range a bit, so maybe he has even fewer value combos.
Expliotatively, idk if ppl bluff enough here though, because you look like you have a flush draw or overpair/Kx and now the flush got here. This could be him fearing u check back Kx or AA and shoving and hoping to get called. That seems likely actually given most likely population tendencies

I have this theory that when a player all of a sudden donk shove the river after he played passively, he has it way more often than he bluffs. Seems that players were donk bluffing river a fair bit a few years ago, nowadays I just don't see it anymore.

Would the spot be appropriate for a bluff in vilain's shoes? Yes, because everything got there and it makes sense to play value hands like that. Does that mean he's taking it? In a vacuum, I'd say, no no no. His line is too suspicious IMO, very few players will hero fold Kx + in this spot.

I call here. Villain doesn't have many value hands that make sense to play like this and not that many hands actually improve on the river that call turn. As an aside, I hope that you're 3 betting this spot a bunch or I don't see the benefit of using the small sizing preflop.

I'd fold, you have many better hands to call with - flushes, straights, trips, Kings full.. Ad blocker seems irrelevant here, having Kd would be more useful.

Are blockers overrated in contemporary holdem?

I agree, I just wonder if that is not overrated over other key factors in the game of poker, such as: is this a spot people take as a bluff often enough? Sometimes blockers will be key in our decisions, sometimes they are more likely to cloud our judgment IMO. Same with calling because 'too high in our range', and other common fallacies (until everybody plays perfect poker lol).

May you share the result please? :-)